In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large bowl, beat the butter and sugar together until light and fluffy. Then add the eggs one at a time, beating well after each addition. Mix in the vanilla extract and lemon zest.

Add the flour mixture in three portions, alternating with two portions of the milk to the batter. Mix in the lemon juice until just combined. Gently fold in the raspberries.

Divide the batter evenly between the prepared cake pans. Bake for 30 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cakes to cool in the pans for 10 minutes, then turn them out onto a wire rack to cool completely.

Mix the cornstarch with 1 tablespoon of water to create a slurry. Set aside.

In a medium saucepan, combine the raspberries, sugar, and lemon juice. Cook over medium heat until the raspberries break down and the mixture comes to a simmer.

Add the slurry to the raspberry mixture. Cook, stirring constantly, until thickened. Remove from the heat and cool completely.

In a large bowl, beat the butter until smooth and creamy. Gradually add the confectioners' sugar, one cup at a time, beating well after each addition. Mix in the lemon zest, lemon juice, vanilla extract, and salt. Beat until light and fluffy.

Place one cake layer on a serving plate or cake stand. Spread the raspberry filling over the top. Place the second cake layer on top. Frost the top and sides of the cake with lemon frosting.