Method
Step 1
Preheat the oven to 190°C/170°C fan/375°F/Gas 5. Grease and line a 5 inch square cake tin.
Step 2
To make the sponge, beat the butter, sugar, egg, flour and lemon zest together until smooth.
Step 3
Transfer the mixture to the greased and lined cake tin, then spread the batter into the corners and smooth the top.
Step 4
Bake for 18–20 minutes, until risen and golden.
Step 5
Once baked, remove from the tin and transfer to a cooling rack. Leave to cool completely.
Step 6
To make the raspberry jam, in a pan set over a medium heat, cook the raspberries, jam sugar and lemon juice until the sugar starts to melt and the raspberries release their juice. Turn the heat up to high and boil the jam until it reaches 105°C/221°F. Transfer to a shallow tray and cool until set.
Step 7
To make the Swiss meringue buttercream, mix the egg whites and sugar in a heat proof bowl, then heat in a microwave for 10 seconds at a time, stirring each time, until they reach 60°C/140°F.
Step 8
Transfer to a stand mixer fitted with a whisk attachment and whisk on high speed until white, fluffy and holding stiff peaks.
Step 9
With the stand mixer running on low, add the butter a little at a time until combined. Add the vanilla bean paste.
Step 10
Transfer to a piping bag and set aside.
Step 11
To assemble the sandwich, using a cake leveller or a serrated knife, carefully slice the sponge horizontally so you have two squares of sponge. Turn the sponges upside down so the fluffy inside is facing down, and the darker outside is facing up.
Step 12
Cut each square diagonally, corner to corner, so you have 4 triangles.
Step 13
Pipe a layer of buttercream on two of the triangles and cover with a layer of jam. Sandwich the remaining triangles of sponge on top to create 2 triangular sandwiches.
