Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the all-purpose flour, cocoa powder, baking powder, baking soda, and sugar. Whisk these dry ingredients together until evenly mixed.
Mix together the milk, melted butter, eggs, and vanilla extract in another bowl. Pour this mixture into the bowl with the dry ingredients, stirring until it transforms into a smooth batter. Pour into a greased cake pan and bake for 20-25 minutes.
Prepare the raspberry mousse while the cake bakes. In a blender, combine the fresh raspberries, 1/4 cup of sugar, and lemon juice. Blend until smooth, then strain through a fine-mesh sieve.