Preheat the oven and line the muffin tin with cupcake liners or a non-stick cooking spray. Melt the chopped chocolate bar with the butter in a large bowl, stirring in 30-second increments.
Add the granulated sugar and brown sugar and combine.
Add the eggs and vanilla extract. Stir to combine.
Add the flour to the chocolate mixture and mix until fully incorporated.
Divide batter between the individual muffin cups in the muffin tin and bake for 18-20 minutes.
While the baked brownie cupcakes are cooling, prepare the chocolate ganache frosting by pouring steaming heavy cream over chocolate chips in a bowl and allowing them to soften in the hot cream for a few minutes before whisking to form the ganache.
Refrigerate the chocolate ganache, covered with plastic wrap, until firm and then beat to achieve fluffy and stiff peaks.
Pipe the chocolate ganache onto the cooled brownie cupcakes and serve!