To make brownie layer, preheat oven to 350ºF. Lightly butter the bottom and sides of a half sheet pan (18-by-13-inches); line with parchment paper, leaving at least an inch of overhang on the two long sides.In a double boiler or in the microwave on low power, melt chocolate until smooth. Set aside to cool slightly.In a separate bowl, sift together flour, cocoa powder, espresso powder, baking powder, and salt and set aside.In the bowl of a stand mixer fitted with the paddle attachment, beat butter, sugar and brown sugar on medium-high speed for 2 to 3 minutes or until light and fluffy. Add eggs, one at a time, beating well after each addition. Increase speed to high and beat for another 2 to 3 minutes or until fluffy and increased in volume. Mix in vanilla and milk.With the mixer running on low, drizzle in in melted chocolate and mix until just incorporated, scraping down the sides of the bowl with a spatula as needed. Sprinkle dry ingredients over top and mix on low speed or fold in with a spatula until just incorporated.Drop dollops of batter onto prepared baking sheet, and spread into an even layer using an offset spatula (it’s ok if the batter doesn’t go quite all the way to the edges, it’ll spread more as it bakes). Bake for 13 to 15 minutes or until top is cracked and edges are slightly darker brown. Place sheet pan on a wire rack to cool completely, then refrigerate for at least 30 minutes (or more) to make the brownie easier to work with.While the brownie is chilling, prepare an 8-by-8-inch square baking pan (ideally one with crisp, sharp corners and straight rather than angled sides). Lining the bottom and sides with two 8-inch-wide by at least 14-inch long strips of parchment paper placed perpendicular in the baking pan, creasing the corners and top edges. You should have a good inch or two of overhang; you will need this later to lift the sandwiches out of the pan.Carefully lift and slide the chilled brownie onto a large cutting board, leaving the parchment on the bottom. Use a ruler to measure and cut two 8-inch squares (should be exactly the same size as the inside of your baking pan). Use a large knife or a sharp rotary cutter to cut through the brownie and the parchment lining the bottom. Any scraps you can do with as you please (chef’s treat!)Gently lower one of the parchment-backed squares into the bottom of the parchment-lined baking pan. Put the other square back in the fridge or freezer until the rest of the sandwich assembly is ready.To make the ganache, place chopped chocolate in a heat-proof bowl. Warm cream in a small saucepan over low heat until it just begins to steam and bubble around the edges (do not let it boil).Pour hot cream over chopped chocolate; let sit for 30 seconds, then gently start to whisk, working in concentric circles from the center out, until chocolate is fully melted and ganache is smooth.  Add butter, one piece at a time, and gently whisk until melted.Immediately pour still-warm ganache over the first layer of brownie in the baking pan, spreading it into the corners as needed, tapping the pan on the counter to level the ganache layer and remove air bubbles. Refrigerate for 2o to 30 minutes until set and no longer shiny.For the ice cream, combine sweetened condensed milk, sifted cocoa powder, and vanilla in a large mixing bowl. In a separate small bowl, dissolve malted milk powder in 1 tablespoon warm water, then mix into cocoa mixture.Pour the cream into a clean metal mixing bowl or the bowl of a stand mixer fitted with the whisk attachment. If you like, you can also chill the metal bowl and whisk which will help your cream to whip up nicely. Turn mixer to medium-high speed and whip until cream forms medium peaks (try not to overmix or it will be harder to incorporate into the cocoa mixture).Spoon in about 1/3 of the whipped cream into bowl with cocoa mixture, folding and gently stirring to lighten up the mixture. Add remaining whipped cream and fold with a large rubber spatula until evenly incorporated and no streaks of bright white cream are visible.Pour ice cream base on top of set ganache, spreading into an even layer. Freeze for 30 to 45 minutes or until ice cream layer starts to freeze and firms up a bit. Gently place second brownie square, parchment side up, on top of ice cream, pressing lightly to adhere (don’t press too hard or the ice cream may squeeze out the sides).Return to freezer and freeze until completely set, at least 4 hours or overnight.Before serving, use the parchment paper overhang to lift the entire block out of the baking pan and onto a cutting board. Use a large knife (running it briefly under hot water to make it easier) to cut sandwiches into 16 even squares. Wipe or rinse of the knife between each slice for the cleanest looking cuts. Serve immediately, or return cut squares to the freezer to enjoy later (if storing longer than a day, put them in an airtight container or wrap individually with plastic wrap). Sandwiches will keep for up to 2 months.