Make the rhubarb compote

• Combine the rhubarb, granulated sugar and lemon juice in a saucepan.
• Cook over medium heat, stirring occasionally, until the rhubarb breaks down into a thick compote.
• Transfer to a bowl and let cool completely.

Bake the vanilla sponge

• Heat the oven to 180 °C and grease and line the cake pans.
• Beat the softened butter and caster sugar until pale and fluffy, then beat in the eggs one at a time with the vanilla extract.
• Fold in the sifted self-raising flour and baking powder, then stir in the milk until smooth.
• Divide the batter between the pans and bake until golden and a skewer comes out clean.
• Cool in the pans for a few minutes, then turn out onto a cooling rack.

Make the whipped white chocolate ganache

• Melt the chopped white chocolate gently in a heatproof bowl.
• Warm part of the cream and stir it into the melted chocolate until smooth, then stir in the remaining cold cream.
• Chill the ganache until cold, then whip it with the icing sugar until light and spreadable.

Assemble the cake

• Place the first sponge layer on a plate and spread over a layer of whipped ganache.
• Add a layer of rhubarb compote, then top with the second sponge layer.
• Cover the cake with the remaining ganache and swirl the top.

Chill and serve

• Chill the cake so the ganache sets before slicing.