Preheat the oven to 180C / 160C Fan / Gas 4.

Butter 2 deep sandwiches tins that are 18 cm in diameter.

See Recipe Notes for adapting the recipe for 20 cm tins.

Sift together the flour, baking powder, and salt then set aside.

In a large bowl and preferably using electric beaters, beat together the butter, sugar, plus vanilla extract if using, until soft and light. Scrape down the sides of the bowl as necessary during this and subsequent steps.

Beat in the eggs one at a time along with approximately 1 tbsp of the sifted flour each time.

Beat in the tablespoon of the milk along with another scant tablespoon of flour.

Gently fold in the rest of the flour.

Divide the mixture between the two prepared tins, ideally weighing for accuracy, and smooth the tops.

Put in the oven and bake until risen and golden brown: when done the edges should be pulling away from the sides of the tin and the tops springy when lightly pressed (approximately 20 min).

Tip: check after 15 minutes and if the cakes are browning very quickly, turn the temperature down a little.

When done, transfer the tins to a wire rack and leave to cool for 5 minutes.

Carefully remove the cakes from the tins and put on the rack until completely cold.

While the cakes are cooling: make the vanilla buttercream.

Beat the butter with a plastic spatula until very soft.

Gradually beat in half the sifted icing sugar, followed by the vanilla extract, then the rest of the icing sugar. Set aside until the cakes are cold.

Turn one of the cakes over so the underside is now uppermost and spread with the vanilla buttercream, making sure it goes right to the edges.

Spread the jam generously over the buttercream, leaving a little bare edge.

Place the second cake on top, right side up, and very gently press down just so the jam starts to peep out.

Put a little icing sugar in a sieve and dust over the top of the cake.

Store in an airtight container. Best eaten within 3 days.