Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a mixing bowl, combine flour, sugar, cocoa powder, baking powder, baking soda, and salt.

Add the egg, buttermilk, vegetable oil, and vanilla extract. Mix until well combined.

Pour the batter into the prepared cake pan and bake for 25 minutes or until a toothpick comes out clean.

Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

In a small saucepan, combine raspberries, sugar, and lemon juice. Cook over medium heat until the raspberries break down and the mixture thickens, about 5-7 minutes.

Remove from heat and let cool.

In a mixing bowl, beat the softened butter until creamy.

Gradually add powdered sugar and cocoa powder, mixing until combined.

Add milk and vanilla extract, mixing until smooth and spreadable.

Once the cake is completely cool, slice it in half horizontally.

Spread the raspberry filling on the bottom layer, then place the top layer back on.

Frost the top and sides of the cake with the chocolate frosting.

Decorate with additional raspberries if desired.