Heat oven to 180C/160C fan/gas 4. Grease and line a 20cm square baking tin. Whisk together the butter, sugar, eggs, flour and custard powder until well combined and fluffy. Spread into the tin and bake for 25-30 mins until golden and risen. Cool in the tin.
Remove cooled cake from tin and spread with the jam. Trim the edges, then cut into 16 squares. Add a meringue to each just before serving.