Step 1

In a saucepan, cook the milk and vanilla over medium-high heat until boiling.

Step 2

In the meantime, whisk together the egg yolks and sugar until pale and smooth, then add the cornstarch and whisk to combine.Tip: The process of whisking the egg yolks and sugar together is called 'blanching'. The sugar protects the egg proteins, preventing lump formation during cooking. This way, your pastry cream will end up perfectly silky-smooth.

Step 3

Pour the hot milk in a slow stream into the egg-sugar mixture, whisking continuously.Tip: Adding the hot milk slowly to the eggs raises their temperature slowly (while also diluting them), preventing scrambling. This process is called 'tempering'.

Step 4

Return the mixture to the saucepan and cook over a high heat with constant whisking until thickened (about 1-2 minutes).

Step 5

Remove from heat and stir in the butter, whisking well until the butter has melted and the cream is smooth and glossy.

Step 6

Transfer the pastry cream into a bowl and cover it with a piece of cling film pressed directly on the surface of the cream to prevent a skin from forming on top.

Step 7

Set aside and allow to cool completely at room temperature. You don't need to chill the pastry cream in the fridge.

Step 8

Adjust the oven rack to the middle position and pre-heat the oven to 350ºF (180ºC). Lightly butter two 6 inch (15cm) round cake tins and line their bottoms with rounds of baking/greaseproof paper.

Step 9

In a large bowl (or the bowl of a stand mixer if using it), sift together the gluten free flour blend, almond flour, sugar, baking powder, xanthan gum and salt.

Step 10

Add the softened butter and, using your fingertips, work it into the dry ingredients until you get a mixture resembling breadcrumbs. You can also use a stand mixer fitted with the paddle attachment or a hand mixer fitted with the double beaters.Tip: This order of adding the ingredients (that is, working the butter into the dry ingredients plus sugar before adding the wet) is known as the “reverse creaming method”. It gives the cakes a very even aeration, a delicate melt-in-the-mouth texture and a uniform rise (with minimal doming).

Step 11

In a separate bowl, mix together the egg, milk and vanilla bean paste, and add them to the flour-butter mixture in two batches, whisking well after each addition, until you get a smooth cake batter with no flour clumps. (Again, you can do this by hand using a balloon whisk, or using a stand mixer fitted with the paddle attachment or a hand mixer fitted with the double beaters.)

Step 12

Evenly divide the cake batter between the two lined cake tins and smooth out the top.

Step 13

Bake at 350ºF (180ºC) for about 22-24 minutes or until well risen, golden brown on top and an inserted toothpick comes out clean.

Step 14

Allow to cool in the cake tins for about 10 minutes before turning them out onto a wire cooling rack to cool completely.

Step 15

If necessary, level off the sponges with a sharp serrated knife.

Step 16

Place the bottom sponge on a cake stand or serving plate of choice.

Step 17

Give the cooled pastry cream a good whisk to smooth it out, then transfer it into a piping bag fitted with a large round nozzle. Pipe the pastry cream (in concentric circles) on top of the sponge, then use a small offset spatula to smooth it out into an even layer that fully covers the sponge. Tip: You could omit the piping and just dollop the pastry cream on top of the sponge and smooth it out, but I’ve found that piping gives a much prettier finish.

Step 18

Place the other sponge on top, with the bottom of the sponge facing upwards (this will give you a nice flat top). Gently press down on it to ensure that it sticks to the pastry cream layer.

Step 19

Place the chopped dark chocolate into a heat-proof bowl.

Step 20

In a saucepan over medium-high heat, cook the double/heavy cream until it only just starts boiling.

Step 21

Pour the hot cream over the chocolate and allow to sit for 2-3 minutes, then stir together until smooth and glossy.

Step 22

Allow it to cool and thicken slightly at room temperature (for about 30 minutes) or in the fridge (for about 15 minutes) with occasional stirring.Tip: Unlike with drip cakes, you don’t want your ganache to be very runny for a Boston cream pie. Instead, the ganache should be thicker, so that the drips down the sides of the Boston cream pie are shorter and wider. For this, you want the ganache to still be runny, yet also thick enough so that it falls off a spatula almost like a “waterfall” rather than a thin drizzle. Also, when the ganache falls off the spatula back into the bowl, it shouldn’t immediately level out and disappear into one chocolate ganache pool – instead, it should pile upon itself.

Step 23

Pour or spoon the ganache onto the top sponge, and use a small offset spatula to smooth it out into an even layer and to guide it to gently drip down the sides. You can create a few swirls on top for a bit of extra decoration.

Step 24

Allow the ganache to set at room temperature for about 30 minutes (or in the fridge for about 15 minutes) before slicing and serving.

Step 25

The gluten free Boston cream pie keeps well in a closed container in a cool dry place (or the fridge) for 3-4 days.