1. Preheat the oven and prepare 2-3 round cake pans by greasing and lining them with parchment paper.
2. In a bowl, whisk together cake flour, baking powder, and salt to combine the dry ingredients.
3. In another bowl, beat butter and sugar until light and fluffy.
4. Add eggs one at a time, mixing well after each addition.
5. Mix in lemon zest, lemon juice, and vanilla extract for flavor.
6. Alternate adding the dry ingredients and buttermilk (or sour cream) to the wet mixture, beginning and ending with the dry ingredients.
7. Fold in fresh raspberries gently to avoid crushing them.
8. Divide the batter evenly between the prepared cake pans.
9. Bake in the preheated oven until a toothpick inserted in the center comes out clean.
10. Let the cakes cool in the pans for a few minutes, then transfer them to wire racks to cool completely.
11. In a saucepan, cook raspberries with sugar and lemon juice until they break down into a thick raspberry compote. Cool completely.
12. If using, make lemon curd by whisking together lemon juice, sugar, eggs, and butter over low heat until thickened, then chill.
13. Prepare the lemon buttercream by beating butter until creamy.
14. Gradually add powdered sugar while mixing until smooth.
15. Mix in lemon juice and lemon zest for flavor. Continue beating until fluffy.
16. If desired, prepare a lemon syrup by mixing lemon juice and sugar, then brush it onto the cake layers.
17. Assemble the cake by placing one cake layer on a serving plate.
18. Spread a thin layer of lemon buttercream over the cake.
19. Add a layer of raspberry compote and, if using, a layer of lemon curd.
20. Place the second cake layer on top and repeat the filling process if making a three-layer cake.
21. Cover the entire cake with lemon buttercream, smoothing out the sides and top.
22. Decorate the cake with fresh raspberries and lemon zest for garnish.
23. Chill the cake briefly to set the frosting before serving.
24. Slice and enjoy your raspberry cake with lemon buttercream!