Preheat your oven to 170ºC (340ºF) and prepare three 8-inch round cake pans by greasing and lining with baking paper.

In a medium bowl, sift together flour, cornstarch, baking powder, baking soda, and salt. Set aside.

In a large bowl, combine sugar and lemon zest, rubbing together until fragrant. Add butter and cream until light and fluffy.

Add eggs two at a time, mixing well after each addition until fully combined.

Gradually add half of the dry mixture, mixing on low speed. Then add sour cream, vegetable oil, lemon juice, vanilla extract, and lemon extract before mixing in the remaining dry ingredients.

Divide the batter among the prepared pans and bake for 20-23 minutes until a toothpick comes out clean.

Cool the cakes on a wire rack for about 10 minutes, then turn them out to cool completely.

Whip butter for the raspberry buttercream until fluffy. Gradually add powdered sugar and raspberry powder until smooth, adjusting with milk if needed.

Assemble the cake by layering raspberry jam and buttercream between each layer, finishing with a crumb coat of buttercream.

Chill the cake for 30 minutes, then frost the entire cake with remaining raspberry buttercream and decorate with raspberries and lemon slices.