Preheat your oven to 180°C (160°C fan).
Grease your traybake tin well.
Line the base with parchment paper.
Put all sponge ingredients in a bowl.
Beat with an electric mixer for 2 minutes.
The batter should be smooth and pale.
Pour the batter into your tin.
Use a spatula to spread it evenly.
Make sure it reaches all corners.
Bake for 35 to 40 minutes.
The cake is done when golden.
It should spring back when pressed gently.
Let the cake cool in the tin.
Then move it to a wire rack.
It must be fully cool before filling.
Beat the softened butter until creamy.
Add icing sugar slowly.
Mix in milk and vanilla.
Cut the cake in half horizontally.
Spread jam on the bottom layer.
Add buttercream on top of the jam.
Place the top layer back on.
Dust with icing sugar.