Preheat oven to 350°F. Butter and flour a 9-inch round cake pan. Melt the stout and butter in a medium saucepan over medium-low heat. Whisk in cocoa and maple syrup until smooth. Cool slightly.
Whisk flour, baking soda, salt and cinnamon together. In another medium bowl, whisk together the eggs, sour cream and vanilla extract until smooth. Carefully add the stout-chocolate mixture to the egg mixture, whisking to combine. Fold in the flour mixture until just combined. Pour batter in pan and bake the cake gives slightly to the touch, 20 to 25 minutes.
Cool cake in pans on a rack for 10 minutes. Remove cake from pan and cool completely. Serve with maple-syrup-sweetened whipped cream.
Recipe adapted from Bon Appétit magazine.