Preheat the oven to 325°F (163°C). Lightly butter the bottom and sides of an 8-inch square metal baking pan. Line the pan with a piece of parchment paper, allowing it to extend up two opposite sides for easy removal later. Butter the parchment paper as well.

In a large heatproof bowl, combine the chopped dark chocolate and butter. Melt them together using a microwave in 20–30 second intervals or over a saucepan of simmering water. Stir until smooth and fully combined. Allow the mixture to cool slightly.

Stir the superfine sugar into the chocolate mixture until fully dissolved. Add the eggs one at a time, whisking thoroughly after each addition until the mixture is glossy and smooth.

Stir in the Baileys Irish Cream. Then, gently fold in the flour, mixing only until no visible streaks of dry ingredients remain. Do not overmix.

Pour the brownie batter into the prepared baking pan and spread it into an even layer using a spatula.

In a separate medium bowl, beat together the softened cream cheese, superfine sugar, egg, and Baileys Irish Cream until smooth and creamy.

Drop spoonfuls of the cheesecake mixture over the brownie batter. Using a knife or skewer, gently swirl the mixtures together to create a marbled effect. Be careful not to over-swirl.

Place the pan in the preheated oven and bake for 35 to 40 minutes, or until the edges are set and a knife inserted into the cheesecake comes out clean. The center may still be slightly soft but will firm up as it cools.

Remove the pan from the oven and allow the brownies to cool completely on a wire rack. Once cooled, use the parchment paper overhang to lift the brownies from the pan. Slice into squares and serve.