To make the blueberry purée and gelée:Combine blueberries, 2 tablespoons sugar, and 1 1/2 teaspoons lemon juice in a small saucepan. Bring to a simmer over medium heat, smashing berries slightly as you stir, until blueberries are softened and bursting.Transfer to a blender or food processor (take caution when blending hot liquids) and purée until completely smooth. You should have 1 cup of blueberry purée.Measure out 1/2 cup of purée and place back into saucepan. Put the remainder in a jar or airtight container and refrigerate it for later (you will need it for the blueberry mousse).Cut gelatin sheets into 1-inch strips and submerge in cool water for at least 5 minutes to soften. If using powdered gelatin, sprinkle 1 1/2 teaspoons over 2 tablespoons of cool water.Add remaining 2 tablespoons sugar and 1 1/2 teaspoons lemon juice, along with 2 tablespoons water to saucepan with purée. Heat until it just barely begins to bubble.Wring out softened gelatin with your hands, squeezing out as much water as you can (for powdered gelatin, just add it straight to the pan, liquid and all). Add to saucepan with warm blueberry mixture and whisk until completely dissolved.Pour blueberry mixture into 1-inch silicone sphere molds. Alternatively, you can use a parchment-lined cake or baking pan (about 8 inches in size). Line with parchment, leaving handles overhanging the sides to allow for easy removal of the set gelatin. You’ll cut ‘coins’ of gelée out of this larger piece, instead of using small half-spheres in your cakes.Refrigerate overnight until completely set, then gently remove from molds (it might help to freeze for 20 or 30 minutes to make the spheres easier to remove).To make the sponge cake:Preheat oven to 350 degrees F. Line a quarter sheet pan (approximately 13-by-9-inches in size) with aluminum foil or a silicone baking mat; grease or butter foil (parchment works as well, though I found it wrinkled the cake a bit).Place egg yolks in a bowl; vigorously whisk with 3 tablespoons of sugar until lightened in color. Whisk in milk and almond extract.Sift in the cake flour and almond flour; add salt and fold until just incorporated.In a clean bowl, whip egg whites until frothy. Add remaining 1 tablespoon sugar and beat until whites hold medium peaks. Fold 1/3 of whites into batter mixture to lighten, then scrape the batter mixture into the bowl with the whites, folding until just incorporated and no pure white streaks remain.Pour batter into prepared sheet pan, spread into a thin, even layer. Bake for 9 to 11 minutes or until cake is spongy to the touch and just barely starting to darken around the edges. Let cool completely.For the blueberry mousse:Cut gelatin into 1-inch strips and submerge in cool water to soften.Meanwhile, rewarm reserved 1/2 cup blueberry purée in a saucepan over medium heat until it is once again smooth (whisk it vigorously to break up any remaining chunks left after refrigeration). Wring out water from softened gelatin and add to warm blueberry purée; whisk until smooth. Set aside and let cool to lukewarm.In a bowl, whisk room temperature mascarpone with sugar, salt and extracts until smooth and paste-like. Add lukewarm blueberry mixture and whisk until smooth.In a clean bowl or the bowl of a stand mixer, beat heavy cream until it holds soft peaks. Add 1/3 of cream to the blueberry mixture and fold to lighten, then add the entire blueberry mixture to the bowl with the cream and fold until fully incorporated.To assemble your cakes, spoon or pipe the mousse into silicone molds, not quite all the way full. Gently press a gelée sphere, curved side down, into the top of each mousse-filled mold. Scrape with an offset spatula to make the top perfectly level with the top of the mold. Finally, cut out rounds of sponge cake exactly the same size as the base of the molds and press onto the top of the mousse. Freeze at least 3 hours or overnight until completely firm.Before you make the glaze, remove cakes from molds and place on a frozen cookie sheet lined with parchment paper. Return to freezer until just before glazing.For Mirror Glaze:Cut gelatin into 1-inch strips and submerge in cool water for at least 5 minutes to soften. Place white chocolate in a heat-proof bowl and set aside.Combine sugar, corn syrup, and water in a saucepan and bring to a boil over medium heat until sugar is completely dissolved and mixture is clear.Remove from heat, then add gelatin (squeeze out as much water as possible first) and whisk until dissolved. Whisk in sweetened condensed milk. Pour hot mixture over chopped white chocolate and let sit for 30 seconds, then stir until chocolate is completely melted and mixture is smooth (you can also use an immersion blender if you have one to expedite this process, just try not to mix air bubbles into the glaze if possible).Let glaze cool, stirring occasionally, until glaze reaches approximately 95 degrees F on an instant-read thermometer. The glaze needs to be the correct temperature or it will not form a thick enough layer on the outside of the cakes.When the glaze reaches 95-96 degrees, add powder or gel food coloring as desired. Darker colors tend to look particularly striking with this glaze technique. You can also divide your glaze up into multiple colors and then swirl them if you’d like.Remove cakes from freezer and place on a parchment lined cookie sheet, elevated off the sheet using small jars or cookie cutters (ideally something smaller in diameter than the cake itself so the glaze can drip off). You may also want to cut out small rounds of cardboard (I used an old bakery box) to place under each cake to make them easier to transfer. If you have more than 6 or 8 individual cakes, I recommend working in two batches. You can always rewarm the glaze if it gets too cool (do so gently over a double boiler or in very short intervals in the microwave).Pour glaze generously over cakes, taking care to ensure there are no bare spots left. Allow the excess glaze to drip off for 5 or 10 minutes, then gently scrape the edges to remove any remaining drops. Carefully transfer cakes using a small offset spatula to cardboard cake rounds or small pieces of parchment paper for easy movement. Any leftover glaze can be scraped up and refrigerated in an airtight container for later use.Glazed cakes will keep in the refrigerator for up to 5 days. Let come to room temperature before serving.