Crush the Oreos into a fine crumb then add the melted butter, mix until you have a wet sand texture all around.
Press into the tart pan and bake for 8 minutes. Set aside while you prepare the filling.
In a medium saucepan whisk together the reduced Guinness, milk, cream, Dorda Liqueur, brown sugar, cocoa powder, and cornstarch.
Bring to a boil then let simmer whisking constantly for 2-3 minutes. Remove saucepan from heat and whisk in the chopped chocolate and vanilla. Whisk until smooth.
Pour on top of Oreo crust. Place a piece of plastic wrap directly on top of the filling to prevent skin from forming. Chill for 3 hours to set the chocolate filling.
Add to Crust & Chill
Whisk together heavy whipping cream, Dorda Liqueur, and powdered sugar until stiff peaks form.
Make Whipped Cream
Spread on top of the chilled tart and use a Microplane zester to grate chocolate on top or drizzle with salted caramel sauce. Store in the fridge for up to three days.
Add Cream to Tart