[Homemade] Maple Cake with Maple-Swirled Sour Cream Frosting

I did a little adapting from various places to get what I wanted, but the approximate recipe is as follows:

Cake * Unsalted butter, softened, for greasing the pan * 344 grams ATK gluten free flour blend * 1 tsp xanthan gum * 1 1/2 teaspoon baking powder * 1/2 teaspoon baking soda * 1/2 teaspoon kosher salt * 1 cup dark-grade pure maple syrup * 1/2 cup canola oil * 1/2 cup minus 2Tbsp whole milk * 3 large eggs * 50 grams light brown sugar * 50 grams granulated sugar * 1/4 cup sour cream * 1 teaspoon vanilla extract

Preheat the oven to 350 degrees with the rack in the middle. Line a 9-inch square pan with buttered parchment paper. In a large bowl, whisk together the flour, xanthan gum, baking powder, baking soda, salt. In another bowl whisk together maple syrup, oil, milk, eggs, brown sugar, granulated sugar, sour cream and vanilla. Stir the wet ingredients into the dry with a rubber spatula just combined and no streaks of flour remain. Pour the batter into the prepared pan. Tap the pan on the counter to release air bubbles. Bake 50-55 minutes, until deep golden and a cake tester or toothpick inserted in the center comes out clean. Let cool in the pan for 5 minutes. Remove from the pan, and let cool completely on a wire rack.

(To make with regular wheat flour: use same amount of flour by weight but omit xanthan gum, reduce baking powder to 1tsp, reduce eggs to 2, and use full 1/2 cup milk. Reduce baking time to 35 minutes.)

Frosting: * 8 Tbsp unsalted butter, softened * 250 grams confectioners' sugar * 3 tablespoons sour cream, at room temperature * 2 Tbsp dark-grade pure maple syrup, plus more for drizzling * 1/2 teaspoon kosher salt

Beat the butter until creamy, 2 to 3 minutes. Gradually add the confectioners' sugar, beating until combined. Add the sour cream, maple syrup and salt, and beat at medium-high speed until smooth, 1 to 2 minutes. Spread the frosting onto the cooled cake. You’ll have extra, so put it on peanut butter cookies or something. Drizzle cake with additional maple syrup and gently swirl together.

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