Butter a large, deep rectangular cake tin (ours was 26 x 37cm) and line with baking parchment. Whisk the egg whites to stiff peaks in a stand mixer or bowl using an electric whisk. With the motor running, add 225g of the caster sugar, a tablespoon at a time, until the sugar has dissolved, about 10-15 mins. Spoon into a separate bowl and wipe the bowl of the stand mixer clean, or set aside.

Heat the oven to 180C/160C fan/ gas 4. Beat the butter, remaining 125g sugar and the vanilla in the stand mixer or a bowl using an electric whisk until light, fluffy and doubled in volume. Beat in the egg yolks, one at a time, mixing well between each addition. Add 4 tbsp milk, then sift in the flour and baking powder, and mix to combine. The batter should be thick enough to drop off a spoon, but if it’s stiff, add another 1 tbsp milk. Spoon the batter into the tin and spread to an even layer. Spread the whipped egg white mix over the batter, then scatter over the flaked almonds.

Bake for 25-30 mins, or until golden and a skewer inserted into the centre comes out clean. Leave to cool in the tin for 15 mins, then remove to a wire rack to cool completely, about 1 hr.

Meanwhile, make the filling. Whisk the egg yolks, 1 tbsp sugar and the cornflour together in a large heatproof bowl, then set aside. Heat the milk, 100ml of the double cream and rum or vanilla together in a saucepan over a low heat until just starting to simmer. Whisk this into the egg yolks in a slow, steady stream, then pour everything back into the saucepan. Stir continuously over a low heat until thickened to a custard and simmering, about 1 min. Pour into a heatproof bowl and set aside to cool. Whip the remaining 250ml double cream with the remaining 1 tbsp sugar for a few minutes until soft peaks form. When the custard has cooled, gently fold in the cream to combine.

Cut the cake in half through the equator using a long, serrated knife. Put one layer, meringue-side down, on a plate or cake board. Spread with the custard cream, then sandwich with the remaining cake layer, meringue-side up. Serve with sliced strawberries, if you like. Will keep chilled for up to two days.