Finely crush the pretzels in a food processor or mini-chopper. Place about 2 tablespoons of the mixture in the bottoms of six 5-ounce jars or beer-tasting glasses.
Place the cream cheese, brown sugar, and cocoa powder in a medium mixing bowl, and cream together until smooth.
With the mixer running on low speed, slowly drizzle in the dark stout beer.
When all of the beer has been incorporated, pour in the cream in a slow and steady stream, while mixing on medium speed.
When all of the cream has been added, turn the mixer up to medium-high and whip until the mixture holds stiff peaks. (Be careful not to overwhip, or the cheesecake mousse could become runny.)
Divide the mixture evenly between the six serving dishes.
Whip the cream and sugar together until the mixture holds soft peaks.
Top the cheesecakes with the whipped cream.
Serve cold.