Combine 1 tablespoon sugar, cornstarch, and raspberries in a saucepan.

Bring the mixture to a boil, stirring continuously.

Cook for 2 minutes until thickened.

Cool the raspberry sauce.

Strain the sauce to remove seeds and set aside.

In a bowl, beat heavy cream, brown sugar, and remaining sugar until stiff peaks form.

Stir in melted chocolate and rum extract.

Gently fold in 2 cups of whipped topping.

Split the angel food cake into three horizontal layers.

Place the bottom layer on a serving plate.

Spread half of the chocolate mixture evenly over the bottom layer.

Place the second cake layer on top of the chocolate mixture.

Spread the remaining chocolate mixture over the second layer.

Top with the final cake layer.

Spread the remaining whipped topping over the top and sides of the cake.

Drizzle 1/4 cup of raspberry sauce over the cake.

Cover the cake and refrigerate until serving.

Serve with the remaining raspberry sauce.