Cake – 1 cup (240 ml) chocolate stout – 1 1/2 sticks (170 g) unsalted butter, chopped – 3/4 cup (75 g) unsweetened cocoa, (I use Dutch processed) – 2 cups (420 g) granulated sugar – 1 teaspoon instant espresso powder – 1/2 teaspoon fine sea salt – 3/4 cup (180 g) sour cream – 2 large egg – 1 teaspoon vanilla extract, or vanilla paste – 2 cups (270 g) all-purpose flour – 2 1/2 teaspoons baking soda Frosting – 17.6 oz (500 g) mascarpone cheese, cold from the refrigerator – 1 cup (240 ml) whipping cream, cold from the refrigerator – 6 tablespoons powdered sugar, plus more to taste – 1 teaspoon vanilla bean paste, or beans from 1 vanilla pod