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Make sugared cranberries:
Bring 1 cup granulated sugar and 1 cup water to a gentle simmer (not a full boil) on the stove, stirring until sugar has dissolved. Remove from heat and add cranberries. Pour mixture into a bowl and let syrupy cranberries chill in fridge overnight, or at least 8 hours. The next morning, drain cranberries (you can reserve syrup for soda or sweetening cocktails). Place remaining 1/3 cup sugar in a bowl and roll cranberries in it. Arrange them on a tray or plate and refrigerate for another 45 minutes to an hour, so that the sugar sets. (They’ll feel mostly dry to the touch.)
Make the cake layers:
Heat oven to 350°F. Butter and flour, or use a nonstick spray to coat three 9-inch round cake pans (see note above re: if you have fewer) and line the bottom of each with a fitted round of parchment paper. Bring stout and molasses to a boil in a large saucepan and remove from heat; whisk in baking soda carefully — it will foam up. Cool to room temperature. In a large bowl, whisk together sugars and oil. Whisk in eggs, then whisk in cooled stout-molasses mixture. Place dry ingredients in a fine-mesh sieve or sifter and shake over bowl. Stir until just combined. Divide batter into prepared cake pans; you’ll have about (updated!) a scant (bit less than) 2 cups or 515 grams of batter in each. Bake for 18 to 22 minutes, until a toothpick inserted into the center of the cake comes out batter-free. Cool in pan on rack for 5 minutes, then flip out onto cooling rack, carefully remove parchment paper (it’s sticky) and flip back right side-up, letting each layer cool completely. You can hasten this along outside (if it’s cold) or in the freezer.
Make whipped mascarpone cream:
Beat heavy cream and powdered sugar in a large bowl with a whisk or electric beaters until soft peaks form. Beat in mascarpone, one spoonful at a time, just until it disappears into the cream.
Assemble cake:
Place first cake layer on cake stand and level top with a serrated knife if it has domed. Spread with 1 cup whipped mascarpone. Repeat twice, then smooth sides. Decorate with sugared cranberries. Serve immediately, or keep refrigerated until needed.
