Preheat the oven to 180°C, fan 160°C, gas 4. Oil two 18cm sandwich tins.
Whisk the eggs and sugar together in a medium bowl for about 1 minute by hand. In a separate bowl, mix the flour, a pinch of salt, baking powder, xanthan gum and cocoa powder together well. In a jug, whisk the oil, milk, vanilla and glycerine (if using) together. Add the dry ingredients to the beaten eggs and sugar, then mix in the wet ingredients.
Divide the mixture between the two oiled tins and bake for about 15 minutes, or until well risen and spongy when pressed lightly in the middle. Cool in the tin for 10 minutes before turning on to a wire rack to cool completely.
For the filling, divide the instant custard equally between two clean bowls, and then pour 150ml boiling water onto each, whisking well. Add the white chocolate to one bowl and the dark chocolate to the other, whisking well to melt the chocolate completely. Set aside to cool until thickened.
Lightly whip the cream with the icing sugar.
Carefully slice each cooled sponge in half widthways. Place one of sponge layers on a serving plate. Spread a third of the dark chocolate custard onto it, then dot with a third of the tinned cherries and fill in the gaps with a little white chocolate custard and whipped cream. Continue layering the sponges and fillings in this way until you reach the top (make sure you save enough whipped cream for the topping). Press the final sponge down lightly.
Spread the whipped cream over the top of the cake, and decorate with some fresh cherries (if using) and a scattering of chocolate sprinkles.
Kitchen secret: to make up a 500g batch of gluten-free flour mix, use 350g fine white rice flour, 100g potato flour and 50g tapioca flour.