Preheat your oven to 180°C (160°C Fan / Gas 4). Grease an 8-inch round cake tin and line the base with baking parchment.

In a small bowl, whisk together the plain flour, baking powder, bicarbonate of soda, and salt.

In a separate mixing bowl, beat the softened butter and caster sugar for 3–4 minutes until pale and creamy.

Beat in the egg until fully incorporated, followed by the sour cream, vanilla extract, lemon juice, and lemon zest.

Fold in the dry ingredients alternating with the milk until you have a smooth, lump-free batter.

Transfer to the prepared tin, level the surface, and bake for 18–22 minutes until golden and a skewer inserted in the center comes out clean.

Allow to cool completely on a wire rack.

In a medium saucepan, whisk together the egg yolks, whole egg, caster sugar, lemon juice, lemon zest, and pinch of salt.

Place over medium-low heat and cook, stirring constantly with a whisk or silicone spatula, for 8–10 minutes until thick enough to coat the back of a spoon.

Take off the heat and pass through a fine-mesh sieve into a clean bowl to ensure a silky-smooth finish.

Stir in the cubed butter until melted and glossy.

Press a sheet of cling film directly against the surface of the curd and chill in the fridge until completely cold.

Sprinkle the powdered gelatine over the 36ml of cold water in a small ramekin and leave to bloom for 5–10 minutes.

Place the chopped white chocolate into a heatproof bowl.

Heat 190ml of double cream in a small saucepan over medium heat until steaming and just beginning to simmer around the edges (do not let it boil).

Remove the cream from the heat, add the bloomed gelatine, and stir until completely dissolved.

Pour the hot cream-gelatine mixture directly over the white chocolate. Let it sit undisturbed for 1 minute, then whisk gently until smooth and fully melted. Let cool to lukewarm/room temperature.

In a separate large bowl, whip the remaining 380ml of cold double cream to medium-soft peaks.

Gently whisk 65g of the cooled lemon curd into the cooled white chocolate mixture.

Fold the whipped double cream into the chocolate mixture in 2–3 additions using a spatula until uniform, light, and airy.

Place the cooled sponge layer into the base of an 8-inch springform tin or cake ring lined with an acetate collar for neat, clean edges.

Pour the white chocolate mousse over the sponge layer, smoothing the top with an offset spatula.

Place into the fridge to chill for at least 2 hours until the mousse is set and firm to the touch.

Gently warm the remaining lemon curd if it has become too stiff, then spread an even layer across the top of the firm mousse.

Return the cake to the fridge to set completely for at least 4 more hours (or overnight).

Remove the springform ring, carefully peel away the acetate collar, and garnish with fresh lemon twists, white chocolate shavings, or fresh berries before slicing.