Preheat the oven to 350°F. Grease and line two 9-inch round cake pans.
Whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt in a large bowl.
In another bowl, whisk eggs, milk, melted butter, and vanilla until well combined. Stir in dry ingredients until smooth, then add hot water and mix until the batter is thin.
Divide batter evenly into prepared pans and bake for 22-25 minutes, until a toothpick comes out clean. Cool completely.
Puree raspberries with sugar and lemon juice; fold into lightly whipped cream to make the mousse. Refrigerate until set.
Heat cream until simmering; pour over chocolate and whisk until smooth glossy ganache forms. Let cool slightly.
Place one cake layer on a serving plate, spread raspberry mousse evenly, top with the second cake layer, then cover with ganache. Chill until set.
Notes
Best served chilled. Store refrigerated for up to 3 days.