Preheat the oven to 325°F (165°C). Thoroughly butter the bottom and sides of an 8-inch metal baking pan. Line the pan with a long strip of parchment paper, allowing it to extend up two sides with overhang to facilitate removal after baking. Butter the parchment paper and set the pan aside.

In a large heatproof bowl, gently melt the butter and dark chocolate together until smooth. Add the superfine sugar and stir until fully incorporated and glossy. Allow the mixture to cool for several minutes.

Add the eggs to the chocolate mixture one at a time, beating well after each addition until the batter appears thick and glossy. Stir in the Irish cream liqueur until evenly combined. Gently fold in the flour, mixing only until no dry streaks remain.

Pour the brownie batter into the prepared pan and spread evenly.

In a separate medium bowl, beat the cream cheese until smooth. Add the superfine sugar, egg, and Irish cream liqueur, and mix until fully combined and creamy. Spoon the cheesecake mixture evenly over the brownie batter in approximately eight dollops. Using a knife or spatula, gently swirl the cheesecake mixture into the brownie layer.

Bake for 35 to 40 minutes, or until both the brownie and cheesecake layers are set. A knife inserted into the cheesecake portion should come out clean, and the edges should appear lightly browned.

Remove from the oven and allow the dessert to cool completely in the pan on a wire rack. Once cooled, lift the cheesecake from the pan using the parchment overhang and slice as desired.