On the stove over a double boiler or in the microwave, melt together the chocolate and butter until smooth and combined. If doing this in a microwave, make sure to melt together in 15 second increments, stirring in between each increment, so the chocolate does not seize. Set aside to cool.
Meanwhile, in the bowl of a stand mixer fitted with the paddle attachment or a medium-sized bowl if using a handheld mixer, beat the cream cheese on medium-high speed until it is fluffy and creamy. It needs to be completely at room temperature to get it smooth, otherwise there will be bigger chunks of cream cheese.
Add the melted chocolate/butter mixture to the cream cheese and beat until combined, scraping down the sides and bottom of the bowl as necessary. Add the powdered sugar and beat on low speed for 10-15 seconds, then turn up to medium speed and beat until combined. Set aside.
In a separate bowl, make the whipped cream. Beat the heavy cream on medium-high speed until stiff peaks form - about 1-2 minutes.
Fold half of the whipped cream into the chocolate/cream cheese mixture until incorporated, then fold in the other half.
Take your pan out of the fridge and pour the chocolate cheesecake filling on top of it, making one smooth and even layer.
Place back in the fridge and let chill for 3 hours or overnight, until the filling has set.
If you are making my classic fudgy brownies (linked above) as garnish, make the brownies according to the recipe instructions while the cheesecake is chilling. Cut up the brownies once COMPLETELY cool, then place on top of the cheesecake.
Slice and enjoy! This is best served chilled. Store in an airtight container in the refrigerator.