Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides.
In a medium bowl, whisk together melted butter and granulated sugar. Add sifted cocoa powder and mix until a thick batter forms. Press evenly into the prepared pan.
Bake the brownie base for 10-12 minutes, until set but slightly soft in the center. Let cool for 5-10 minutes.
In a large bowl, beat room-temperature cream cheese with an electric mixer until smooth. Add sugar, an egg, and vanilla extract; beat on low just until combined.
Pour cheesecake filling over the slightly cooled brownie base and spread evenly. Bake for 25-30 minutes, until edges are set and center has a slight jiggle.
Cool in the pan on a wire rack for 1 hour, then cover and refrigerate for at least 4 hours or overnight until firm.
Place chopped dark chocolate in a heatproof bowl. Heat heavy cream until just simmering, pour over chocolate, let sit 2 minutes, then stir until smooth.
Lift chilled cheesecake out using parchment. Spread ganache on top, sprinkle with mini chocolate chips, and slice into 16 squares.