Preheat the oven to 180°C, fan 160°C, gas mark 4. Lightly brush the base and sides of 2 x 20cm sandwich tins with a little oil, then line the bases with baking paper.
Cream the butter and sugar together in a large mixing bowl with a wooden spoon or electric mixer until light and fluffy. Stir in the lemon zest.
Sift the flour and baking powder together into a separate bowl.
Gradually beat alternate spoonfuls of beaten egg and flour into the creamed butter mixture until everything is added and the mixture is smooth.
Divide the mixture evenly between the tins and level the top.
Place on the middle shelf in the oven and bake for 20 minutes until well risen, golden and the cake springs back when lightly pressed.
Allow to cool in the tins for a few minutes, then loosen the edges with a round-bladed knife, turn the cakes out and leave to cool completely.
Mix most of the sliced and hulled strawberries into the jam and softly whip the cream.
Place the bottom cake on a serving plate, then spread with the whipped cream, then the jam mix.
Place the remaining cake on top, and decorate with the 4 strawberry halves and a sprinkling of caster sugar.
Cook's tip: If the jam is very set, warm it a little in the microwave first before adding the strawberries to make it easier to mix together.