Preheat the oven to 180°C/fan 160°C/gas mark 4. Grease and line two 20cm cake tins with greaseproof paper. Sift the flour, custard powder and baking powder into a bowl and set aside.
Place the soft butter in a large bowl with the sugar and beat for 5-6 minutes, until pale and fluffy. Add the eggs one by one and beat until smooth, then stir through the flour mixture. Add the milk, then divide the mixture between two cake tins.
Gently heat 2 tbsp jam in a small saucepan over a low heat. Dollop this in small sections all over the surface of the two cakes. Using a toothpick or fine skewer, run these through the jam to create a swirled effect. Bake the cakes for 25-30 minutes, until risen and golden and a skewer comes out clean when inserted in the middle. Leave to cool in the tins for 10 minutes, then transfer to a wire rack and leave to cool completely.
Place the butter and half the icing sugar in a bowl and beat with a handheld electric mixer until incorporated. Gradually add the remaining icing sugar with the custard powder. Add the milk and continue to beat for a further 2 minutes, until smooth and aerated. Once the cakes have completely cooled, spread the remaining jam over the surface of one of the cakes, then spread the buttercream over the jam. Top this with the remaining cake and serve.