Directions for cheesecake batter:
1. Place the cream cheese, sugar, egg yolk and Baileys Irish Cream in the bowl of a food processor and process until smooth.
Directions for Brownies:
1. Preheat the oven to 350 degrees. Lightly grease an 8-by-8-inch baking pan with cooking spray. To make removing the brownies easy, line the pan lengthwise and widthwise with two 7-by-12-inch sheets of parchment paper or aluminum foil, shiny side up, and use the overhang as handles.
2. Place the chocolate in a small heatproof bowl and set in a wide pan or skillet of hot water. Set aside for 5 minutes, stirring 4 to 5 times, and let it melt completely. Stir until smooth. In a medium bowl, whisk the flour, cocoa, baking powder, and salt until well blended. Set aside.
3. With a stand or hand mixer set on low speed, beat the butter and sugar in a medium bowl until well blended. Beat in the eggs, one at a time, until fully blended, scraping down the sides and bottom of the bowl as necessary. Beat in the vanilla until blended. Alternately blend the dry ingredients and the melted chocolate into the egg mixture in increments. Using a rubber spatula or wooden spoon, fold in the walnuts. Stop when the ingredients are just blended. Scrape the batter into the prepared baking pan and spread evenly. Set aside.
4. Bake about 35 minutes, until a tester inserted in the center comes out clean or with a few crumbs clinging to it. Transfer to a wire rack and let the brownies cool completely, then lift from the pan using the overhanging parchment. Cut into squares.
5. Place large spoonfuls of the cheesecake mixture on top of the chocolate mixture and, using a butter knife, swirl to partially combine.
6. Bake for 45-50 minutes or until set (do not over-bake).