Line 2 cake tins (20cm diameter) with baking paper and preheat an oven to 180°C. Beat the butter and sugar together in a large bowl or kitchen mixer until light and fluffy. Crack in the eggs one at a time as you beat, including a heaped tablespoon of the flour every 2 eggs. Sieve the rest of the flour into the bowl with the baking powder and gently mix it into the wet ingredients, folding to avoiding knocking out all the air you’ve just beaten in. Fold in the buttermilk and mango purée.
Divide the mixture between the cake tins. Bake for 35 minutes. Remove the cakes from the oven once baked, risen and when the sponge bounces back when lightly pressed. Leave to cool completely, on a wire rack. Peel back the baking paper and trim each cooled sponge so that they’re neat and even.
Lower the oven to 150°C. Sift together the ground almonds and the powdered sugar into a mixing bowl. Crush the digestives to a fine crumb and stir them into the almond mix. Tip 30g of the egg whites into a mixing bowl. Tip the sugar into a small saucepan and pour over just enough water to cover it. Heat the water and sugar in a saucepan until it reaches 110°C then begin to whip the egg whites to soft peaks. When the sugar syrup reaches 118°C, slowly pour it into the egg whites to form a meringue. Continue to whisk until the bowl is no longer hot to touch.
Beat the dry ingredients into the meringue and loosen with the second set of egg whites. You want to deflate the meringue, so don’t worry about being too gentle, but don’t overwork the mixture so that it goes runny. You want something that just holds its form for a few seconds before sinking back slightly.
Grab a piping bag with a round tip and fill it with the macaron mixture. Line a tray with a silicon baking mat or baking paper. Pipe a disc about 20cm in diameter. Leave for about 45 minutes, until the mix is dry to touch. Bake in the oven for 10-12 minutes. Leave to cool completely and trim to desired size.
Put a medium saucepan over a medium heat and pour in the milk and condensed milk. Bring to a simmer, stirring to avoid it catching on the bottom. In a mixing bowl whisk together the egg yolks, flour, and cornflour. Pour the hot milk over the yolks slowly, whisking constantly. Return the mix to the saucepan and put it back over a low heat. Stir until it thickens up, strain through a sieve and stir in the butter to create a pastry cream. Leave to cool completely.
Whip the cream to stiff peaks then add 1⁄3 of it to the pastry cream. Fold until fully combined then add the remaining two thirds with the mango pureé and fold again until fully combined.
Line a baking tray with baking paper and place a cake ring (20cm diameter) on top. Grease the sides of the ring and line the sides with more baking paper. Trim the baking paper so that it just reaches the top of the ring then place the macaron into the cake ring. Pipe a layer of the mango cream over the macaron and place one of the sponges on top. Pipe another layer of mango cream over the sponge and around the sides between the sponge and the cake ring. Add the second sponge and fill in the rest of the ring with mango cream.
Freeze until completely firm then remove from the freezer 20 minutes before you want to dig in. Garnish with digestive crumbs on the sides and sliced mango on top.