Preheat the oven to 350 degrees F. Lightly butter six 8-ounce ramekins and set on a baking sheet; set aside. Put the butter and milk in a medium microwave-safe bowl and microwave in 30-second intervals until the butter is melted. Add 1/2 cup granulated sugar, the vanilla and salt and whisk until smooth. Add the flour and baking powder and whisk until smooth. Divide among the prepared ramekins.
Combine the remaining 1/2 cup granulated sugar, the brown sugar, cocoa power and espresso powder in a small bowl; sprinkle over the batter. Pour the hot water over the batter in the ramekins (about 3 tablespoons per ramekin); do not stir.
Bake until a toothpick inserted into the centers comes out clean, about 30 minutes. Remove from the oven and brush the tops with the liqueur, if desired. Lightly dust the top of each with cocoa powder and confectioners' sugar. Let stand at least 30 minutes before serving.