Heat oven to 325°F. Wrap a 9-inch springform pan with heavy foil. Grease and line the bottom with parchment.
Make the brownie base: Whisk melted butter, granulated sugar, and brown sugar until glossy.
Whisk in 2 eggs and 1 tsp vanilla. Stir in cocoa, flour, and salt until just combined. Fold in chocolate chips.
Spread batter in the pan. Bake 16–18 minutes until set at edges but still soft in center. Cool 10 minutes.
Boil a kettle for a water bath. Place a roasting pan on the middle rack.
Make the filling: Beat cream cheese on medium until smooth. Add sugar, cornstarch, and a pinch of salt; beat to combine.
Mix in sour cream, Irish cream, and vanilla. Beat in 3 eggs one at a time on low, scraping the bowl as needed.
Pour filling over the warm brownie base. Set pan in the roasting pan and add hot water halfway up the sides.
Bake 55–60 minutes until edges are set and center still jiggles slightly. Turn off oven; crack door and rest 1 hour.
Remove from water bath. Cool to room temperature, then chill until fully cold, at least 6 hours or overnight.
Make ganache: Heat cream until steaming. Stir in Irish cream. Pour over chocolate chips, rest 2 minutes, then whisk smooth.
Spread ganache over the chilled cheesecake. Chill 20–30 minutes to set.
Whip cold cream with powdered sugar to soft peaks, if using. Unmold cheesecake, slice, and garnish with whipped cream.