Beat the butter with the caster sugar and remaining 100g of brown sugar until fluffy, then beat in the eggs and vanilla. Fold in the flour, ground almonds, baking powder, a pinch of salt and the lemon zest. Spoon this over the peaches, then smooth the top, being careful not to disturb the layer of peaches underneath. Bake for 1 hour-1 hour 15 minutes, or until a skewer inserted into the middle comes out clean. Cool in the tin for 10 minutes, then carefully invert onto a wire rack to cool completely.