Prepare the Cake Layers: Preheat oven to 350°F (175°C). Grease and flour two 8-inch round cake pans. In a medium bowl, whisk together the flour, granulated sugar, baking powder, and salt.

Cream Butter and Eggs: In a large bowl, using an electric mixer, cream the softened butter until light and fluffy (about 2-3 minutes). Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Combine Wet and Dry for Moisture: Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the milk, beginning and ending with the dry ingredients. Mix just until combined after each addition, being careful not to overmix. Overmixing develops gluten and can lead to a tough, dry cake.

Bake Cake Layers Properly: Divide the batter evenly between the prepared pans. Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean. Do not overbake, as this is another key cause of dry cake. Remove from oven and let cool in pans for 10 minutes before inverting onto a wire rack to cool completely.

Make Raspberry Filling: In a small saucepan, combine raspberries, granulated sugar, and lemon juice. Cook over medium heat, stirring occasionally, until raspberries break down and mixture begins to simmer (about 5-7 minutes). Stir in the cornstarch slurry and cook for another 1-2 minutes, stirring constantly, until thickened. Remove from heat and let cool completely.

Prepare Vanilla Buttercream: In a large bowl, cream the softened butter with an electric mixer until light and fluffy. Gradually add the sifted powdered sugar, one cup at a time, mixing well after each addition. Beat in the vanilla extract and salt. Add milk or cream one tablespoon at a time until the frosting reaches a smooth, spreadable consistency.

Assemble the Cake: Once cake layers are completely cool, place one layer on a serving plate. Spread about half of the raspberry filling evenly over the top, leaving a small border. Spread a thin layer of buttercream over the filling, then place the second cake layer on top. Spread the remaining raspberry filling, then apply a thin crumb coat of buttercream to the entire cake. Chill for 15-20 minutes to set the crumb coat.

Final Frosting: Apply the remaining buttercream evenly over the top and sides of the cake. Decorate with fresh raspberries if desired. Slice and serve!