This is my go to filling for my favorite chocolate cake. They key to it is making sure it sets up completely before using it or else your layers will split apart. Make this before you even make the cake to give it time to set up.
1/2 cup hot water
4 tbsp cocoa powder
1 1/2 cups semi-sweet chocolate chips
2 cups heavy cream
2 tbsp sugar
1. Dissolve cocoa powder in hot water.
2. Melt chocolate chips in the microwave until smooth.
3. Add cocoa powder mixture to melted chocolate chips.
4. In a chilled metal bowl using a chilled metal whisk, beat the heavy cream and sugar until stiff peaks form.
5. Fold melted chocolate mixture into whipped cream until well combined.
6. Refrigerate until ready to use.