Heat the oven to 180C/160C fan. Grease two 20cm sandwich tins and line the bases with baking paper.
Put the baking spread, caster sugar, self-raising flour, baking powder, eggs, vanilla and milk in a large bowl and beat with an electric whisk for 2-3 minutes until pale, fluffy and smooth. Scrape down the bowl halfway so no pockets of flour remain.
Divide the mixture evenly between the tins and smooth the tops. A quick few taps of each tin on the worktop settles the batter.
Bake for 22-25 minutes until golden, springy to a gentle press and shrinking slightly from the sides of the tins. Cool in the tins for 10 minutes, then turn out onto a wire rack to cool completely.
For the filling, beat the softened butter until pale and creamy, then sift in the icing sugar and beat again until light and fluffy.
Spread the buttercream over one cooled sponge, spoon the jam over the buttercream and spread gently, then sandwich the second sponge on top. Dust the top with icing sugar just before serving.