Blackberry Velvet Gothic Cake

2. Make the Blackberry Filling:
In a small saucepan, combine the blackberries, sugar, and lemon juice over medium heat. Stir frequently until the berries break down (about 5-7 minutes).
In a separate bowl, whisk together the cornstarch and water until smooth. Add this mixture to the saucepan and stir constantly until the filling thickens (about 1-2 minutes).
Remove from heat and let the filling cool completely.
3. Prepare the Chocolate Ganache:
In a small saucepan, heat the heavy cream until it just begins to simmer (but do not let it boil).
Remove from heat and pour the cream over the chopped chocolate in a heatproof bowl. Let it sit for 2-3 minutes to soften the chocolate.
Stir gently until the chocolate is fully melted and smooth. If using, stir in the butter for extra shine. Let the ganache cool slightly until it thickens but remains pourable.