Heat oven to 190C/fan 170C/gas 5. Butter two 20cm sandwich tins and line with non-stick baking paper. In a large bowl, beat all the cake ingredients together until you have a smooth, soft batter. Fold in the blueberries through the raw batter before baking.
Divide the mixture between the tins, smooth the surface with a spatula or the back of a spoon, then bake for about 20 mins until golden and the cake springs back when pressed. Turn onto a cooling rack and leave to cool completely.
To make the filling, beat the butter until smooth and creamy, then gradually beat in icing sugar. Beat in vanilla extract if you’re using it. Spread the butter cream over the bottom of one of the sponges, top it with jam and sandwich the second sponge on top. Dust with a little icing sugar before serving. Keep in an airtight container and eat within 2 days.
For a simple filling, stir the caster sugar into the clotted cream. Sandwich the sponges with the blueberry conserve and a third of the clotted cream filling. Top with remaining cream and decorate with blueberries.