Vanilla and Raspberry Mousse Cake (Entremets)

Whisk and bake the sponge. Heat the oven to 180°C (160°C fan) for at least 20 minutes, then line and grease the 20 cm tin. Whisk the eggs and sugar on high speed for 5-6 minutes until thick enough to leave a ribbon trail when the whisk is lifted; this traps the air that gives the sponge its lift. Fold in the sifted flour, then the melted butter loosened with a spoonful of batter, keeping the mixture as airy as possible. Spread into the tin and bake for 12-15 minutes until golden and springy to the touch.

Cool and trim the sponge. Leave the sponge to cool completely in the tin before turning out, as a warm sponge will tear when cut. Trim it down to an 18 cm round, keeping it narrower than the cake tin all round. This gap matters: it lets the mousse flow right down the sides for a clean edge once sliced. Set the trimmed sponge aside until assembly.

Set the raspberry middle. Sprinkle the gelatine over the cold water and leave for 5 minutes to bloom, which lets the granules swell evenly so they melt without clumps. Meanwhile simmer the raspberries, sugar and lemon juice together for 5 minutes until soft and juicy. Press the mixture through a fine sieve to remove the seeds, then stir in the bloomed gelatine until fully dissolved in the warm fruit. Pour into the cling-film-lined 16 cm ring and freeze for 2 hours until firm enough to hold its shape.

Make the vanilla custard base. Bloom the second batch of gelatine in its cold water. Heat the milk until steaming, whisk it into the beaten egg yolks and sugar, then return everything to the pan and stir constantly over low heat until it coats the back of a spoon and reaches 82°C; do not let it boil or the yolks will scramble. Off the heat, stir in the gelatine and vanilla, then pour the hot custard over the chopped white chocolate and stir until smooth. Cool the mixture, stirring occasionally, to 28-30°C so it is cold enough not to melt the whipped cream but still fluid enough to fold through evenly.

Fold in the whipped cream. Whip the double cream to soft peaks that hold their shape but still look glossy. Fold it into the cooled custard in two additions, cutting through gently to keep as much air in as possible. Working in two stages stops the dense custard from knocking all the air straight back out of the cream. The result should be a pale, light mousse that holds a soft shape.

Assemble the cake upside down. Set the 20 cm ring with its acetate collar on a flat tray and pour in two-thirds of the mousse. Press the frozen raspberry disc into the centre, then cover with the remaining mousse. Finish with the trimmed sponge, cut side down, pressing it level; building the cake upside down means the sponge ends up on the bottom once turned out, giving a neat flat top. Freeze overnight until completely solid.

Make the raspberry glaze. Bloom the gelatine for the glaze in its measured water. Simmer the raspberries, sugar and water for 5 minutes, then sieve out the seeds and stir in the gelatine until dissolved. Cool the glaze to 30-32°C, stirring now and then; at this temperature it is still pourable but thick enough to cling to the frozen cake rather than sliding straight off.

Unmould and glaze the cake. Turn the frozen cake upside down onto the wire rack set over a tray and peel away the acetate collar. Pour the glaze over in one steady go so it coats the top and runs evenly down the sides, using the frozen surface to set the glaze almost instantly into a glossy shell. Lift the glazed cake onto a serving plate.

Thaw before slicing. Move the cake to the fridge and let it thaw for 4-5 hours, as a frozen centre needs this long to turn soft and mousse-like again. Slice with a knife run under hot water and wiped dry between cuts for clean layers. Keep the cake refrigerated until ready to serve.