Preheat oven to 160ºC (320ºF). Place the flour, baking powder, sugar, lemon rind and butter in the bowl of a food processor and process until the mixture resembles fine breadcrumbs. With the motor running, add the egg yolks and process until well combined. Press into the base of a 20cm-square cake tin lightly greased with butter and bake for 20–25 minutes or until light golden and firm to the touch.
To make the mascarpone topping, place the mascarpone, sugar, egg, flour, lemon rind and vanilla in the bowl of a food processor and process until just smooth. Spoon onto the base and bake for 30–35 minutes or until the edges are golden and the middle is just set. Refrigerate for 3 hours or until set. Slice and serve with double cream. Serves 8.