Preheat your oven to 170ºC (340ºF) and prepare three 20 cm (8-inch) round cake pans by greasing them.

Sift together the all-purpose flour, cornstarch, baking powder, baking soda, and salt in a bowl.

In a large mixing bowl, combine granulated sugar and lemon zest. Cream in room temperature butter until light and fluffy.

Add eggs to the mixture, two at a time, and mix until combined. Gradually mix in half of the dry ingredients.

Blend in sour cream, vegetable oil, lemon juice, lemon extract, and vanilla extract until smooth, then add remaining dry ingredients.

Divide batter among prepared pans, smoothing tops. Bake for 20-23 minutes until a toothpick comes out clean.

Cool the cakes in pans for 10 minutes, then transfer to a wire rack to cool completely.

For frosting, sift powdered sugar and raspberry powder. Beat butter until fluffy, then mix in sugar and milk until smooth.

Assemble the cake by placing one layer on a serving plate, add raspberry buttercream, followed by raspberry jam, and repeat.

Chill the assembled cake for about 20 minutes. Spread remaining buttercream over the cake and decorate with raspberries and lemon slices.