Preheat the oven to 190°C/fan 170°C/gas mark 5 5. Line two 20cm sandwich tins with non-stick baking paper and butter them.
200g caster sugar, 200g softened butter, 4 beaten eggs, 200g self-raising flour, 1 tsp baking powder, and 2 tbsp milk, combined in a large mixing bowl until smooth and soft batter.
Divide the batter among the muffin tins and smooth the tops with a spatula or the back of a spoon.
Bake for about 20 minutes, or until golden and springy to the touch.
Transfer to a cooling rack to cool entirely.
To create the filling, mix together 100g softened butter and 140g sifted icing sugar until smooth and creamy, then add a drop of vanilla essence (if using).
Cover the bottom of one of the sponges with buttercream. Sandwich the second sponge on top of the 170g strawberry jam.
Before serving, dust with a little icing sugar. Eat within 2 days if stored in an airtight container.