Ingredients
For the cake
Unsalted butter, softened, for greasing the pan
2 1/2 cups (344 grams) flour, plus more for dusting the pan
1 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon kosher salt
1 cup Plew Farm maple syrup
1/2 cup canola oil
1/2 cup whole milk, at room temperature
2 large eggs, at room temperature and lightly beaten
1/4 cup firmly packed light brown sugar
1/4 cup granulated sugar
1/4 cup sour cream, at room temperature
1 teaspoon vanilla extract
For the frosting
8 tablespoons (1 stick) unsalted butter, softened
2 cups confectioners' sugar
3 tablespoons sour cream, at room temperature
2 tablespoons Plew Farm maple syrup, plus more for drizzling
1/2 teaspoon kosher salt
Directions
Make the cake: Preheat the oven to 350 degrees with the rack in the middle. Butter and flour a 9-inch square baking pan. Line the pan with parchment paper, letting excess extend over the sides of the pan (the overhang will give you handles to lift the cake out after it's baked).
In a large bowl, whisk together the flour, baking powder, baking soda and salt. Add the maple syrup, oil, milk, eggs, brown sugar, granulated sugar, sour cream and vanilla, and stir with a rubber spatula just until combined and no streaks of flour remain. (There will be some lumps in the batter; this is okay.) Pour the batter into the prepared pan. Tap the pan on the counter eight times to release air bubbles.
Bake 30 to 35 minutes, until deep golden and a cake tester or toothpick inserted in the center comes out clean, covering with foil after 20 minutes to prevent excessive browning. (It may dip in the middle, don't worry.) Let cool in the pan for 5 minutes. Using the parchment overhang as handles, remove from the pan, and let cool completely on a wire rack.
Make the frosting: In the same (cleaned-out) bowl you used to make the cake, beat the butter with a handheld mixer on medium speed until creamy, 2 to 3 minutes. (You can do this in a stand mixer with the paddle attachment, if you prefer.) 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. Do not overbeat.
Spread the frosting onto the cooled cake. Drizzle with additional maple syrup and serve.