Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans and line the bottoms with parchment paper.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Mix well.
Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat with an electric mixer on medium speed for 2 minutes.
Carefully stir in the boiling water until combined; the batter will be thin.
Divide the batter evenly between the prepared cake pans. Bake for 30 minutes, or until a toothpick inserted in the center comes out clean.
While the cakes are baking, prepare the raspberry filling. In a saucepan over medium heat, combine the raspberries, sugar, cornstarch, and lemon juice. Cook until the mixture starts to thicken, about 5-7 minutes. Set aside to cool.
Allow the cakes to cool in the pans for 10 minutes before transferring them to wire racks to cool completely.
For the frosting, beat the softened butter with an electric mixer until creamy. Gradually add the powdered sugar and cocoa powder, mixing until combined. Add the vanilla extract and milk, and beat until the frosting is smooth and fluffy.
Once the cakes are completely cool, place one cake layer on a serving plate. Spread half of the raspberry filling over the top, then top with the second layer of cake.
Frost the top and sides of the cake with the chocolate frosting. Optionally, garnish with remaining raspberries.
Slice and serve!