Preheat oven to 325°F. Line an 8x8-inch baking pan with foil or parchment paper, leaving a slight overhang on two sides. Using a food processor, pulse the graham crackers, pretzels, and sugar until finely ground. Add the butter and pulse until mixture is evenly moistened. Press the mixture into the bottom of the prepared pan.
For the filling:
In a small saucepan combine the chocolate and beer over medium-high heat. Heat until the chocolate is melted, stirring constantly. Remove from heat and allow to cool.
Wipe the processor bowl clean and add the cream cheese and sour cream. Process until smooth, scraping sides of the bowl as needed. Add both sugars, cocoa powder, vanilla and espresso powder and process until well combined. Scrape down the sides of the bowl and add the egg and cornstarch and pulse to combine. Add the melted chocolate mixture and pulse until combined. Pour the filling over the cookie crust.
Bake for 35 to 40 minutes, or until the edges of the cheesecake are set and the center is only slightly jiggly. Remove to a wire rack and cool completely. Cover and chill in the refrigerator for at least 2 hours, but preferably overnight. Use the foil to lift the cheesecake from the pan and place on a cutting board. Using a sharp knife, slice into bars. For cleaner cuts, dip knife in hot water and wipe between cuts.