Reduce oven temperature to 325°F.

While the crust is baking, make the filling.

In a small saucepan, heat the chocolate and beer over medium-high heat, stirring constantly until melted. Remove from heat and set aside to cool to room temperature.

In a large bowl, use an electric mixer fitted with a paddle attachment to beat the cream cheese, granulated sugar, and brown sugar on medium-high speed until smooth and creamy, at least 2 minutes, scraping down the sides and bottom of the bowl often.

Add the sour cream and cocoa powder and beat until combined. Pour in the cooled chocolate mixture and mix until incorporated. Add the eggs one at a time, blending after each addition until just combined. Scrape down the sides and bottom of the bowl, then mix in the vanilla, espresso powder, and salt until just combined. Don’t over-mix.

Pour the batter evenly into the cooled crust. Tap the pan on the counter a few times to release any bubbles.

Bake at 325°F for about 35 minutes, or until the edges are set and the center has a slight jiggle. Turn off the heat and crack the oven door slightly, letting the cheesecake rest inside for 10 minutes. Transfer the entire pan to a wire rack to cool completely, then cover and refrigerate for at least 6 hours or overnight.

When ready to serve, make the topping. In a mixing bowl fitted with the whisk attachment, whip the heavy cream on medium-high speed until soft peaks form, about 4 minutes. Add sifted powdered sugar and mix on low speed until combined. Scrape down the bottom and sides of the bowl, then add the stout and whip on medium-high speed until medium peaks form, about 1 to 2 minutes. Transfer the whipped cream to a piping bag fitted with a star tip.

Use the parchment overhang to lift the cheesecake out of the pan. Place on a cutting board and slice into bars using a sharp knife. For cleaner cuts, wipe the knife with a warm, damp cloth between slices.

Pipe rosettes onto each slice and sprinkle with cocoa powder, if desired. Serve immediately.