Heat oven to 180C/160C fan/gas 4 and line the base and sides of a 20cm square tin with baking parchment (the easiest way is to cross 2 x 20cm-long strips over the base). To make the almond sponge, put the butter, sugar, flour, ground almonds, baking powder, eggs, vanilla and almond extract in a large bowl. Beat with an electric whisk until the mix comes together smoothly. Scrape into the tin, spreading to the corners, and bake for 25-30 mins – when you poke in a skewer, it should come out clean. Cool in the tin for 10 mins, then transfer to a wire rack to finish cooling while you make the second sponge.