Preheat the oven to 350 degrees F. Spray a 12-cup bundt pan with cooking spray and dust with cocoa powder, tapping out the excess.
In a medium bowl, whisk together the flour, baking soda and salt. Reserve for later use.
In a small saucepan, combine the butter and beer. Cook over medium heat until the butter melts. Remove from the heat and add 8 ounces of the chocolate, whisking until the chocolate completely melts and the mixture is smooth.
Using an electric mixer set to medium-high speed, beat the egg, brown sugar and granulated sugar until fluffy. Beat in the chocolate-stout mixture and sour cream. Reduce the speed to low and gradually add the flour mixture until just combined, making sure not to over mix.
Pour the batter into the prepared pan and bake until a toothpick inserted into the center comes out clean, about 50 minutes.
Cool the cake for 30 minutes in the pan and then invert onto a serving plate.
In a small saucepan, bring the whipping cream and Mexican crema to a boil. Once boiling, remove from the heat and add the remaining 4 ounces chocolate. Whisk until smooth. Drizzle over the inverted bundt cake. Using a fine mesh sieve, sprinkle some powdered sugar over the top. Arrange fresh mint leave around the cake and serve.