For the cake
2 cups (280 g) all purpose gluten free flour (I used Better Batter)
1 teaspoon xanthan gum (omit if your blend already contains it)
1/4 cup (36 g) cornstarch
1 1/2 teaspoons baking powder
1/2 teaspoon baking soda
3/4 teaspoon kosher salt
3/4 cup (150 g) granulated sugar
Zest of 2 lemons, finely grated
1/2 cup (109 g) packed light brown sugar
12 tablespoons (168 g) unsalted butter, at room temperature
3 eggs (150 g, weighed out of shell) at room temperature, beaten
1/4 cup (2 fluid ounces) freshly-squeezed lemon juice (from about 2 medium-size lemons)
3/4 cup (180 g) sour cream, at room temperature
1 1/4 cups (150 g) fresh blueberries, tossed with 1 teaspoon cornstarch (plus a few more for decoration, optional)
For the lemon buttercream filling
16 tablespoons (224 g) unsalted butter, at room temperature
3 1/2 to 4 cups (400 to 460 g) confectioners’ sugar
1/2 teaspoon kosher salt
3 to 4 tablespoons freshly squeezed lemon juice (from about 2 medium-size lemons)
For the rich lemon glaze
2 to 2 1/2 cups (230 to 290 g) confectioners’ sugar
1/8 teaspoon kosher salt
3 tablespoons freshly squeezed lemon juice (from about 1 large lemon)
2 tablespoons (28 g) unsalted butter, at room temperature
First, make the cake. Preheat your oven to 350°F. Grease and line two 9-inch square (or round) cake pans and set them aside. In a large bowl, place the flour, xanthan gum, cornstarch, baking powder, baking soda, salt and granulated sugar, and whisk to combine well. Add the lemon zest, and whisk into the dry ingredients to ensure that the lemon zest isn’t clumped. Add the brown sugar and whisk again, working out any lumps. Create a well in the center of the dry ingredients and add the butter, eggs, lemon juice, and sour cream, mixing to combine after each addition, and mixing until smooth after the final addition. The batter will be thick. Add the blueberries tossed in cornstarch, and fold carefully into the batter until they’re evenly distributed throughout. Divide the cake batter evenly among the two prepared pans and spread into an even layer. Place in the center of the preheated oven and bake until a toothpick inserted in the center of each cake comes out clean (20 to 25 minutes), rotating once during baking. Remove the pans from the oven and allow the cakes to cool in the pans for 10 minutes before transferring to a wire rack to cool completely.
While the cakes are cooling, make the buttercream filling. Place the butter in the bowl of your stand mixer fitted with the paddle attachment (or a large bowl with a handheld mixer) and beat on medium speed until the butter is light and fluffy. Add 3 1/2 cups of the confectioners’ sugar and the salt, and mix on low speed until the sugar is absorbed into the butter. Increase the speed to medium-high, add the salt and lemon juice, and continue to beat until the frosting is light and fluffy (about 3 minutes). Add more confectioners’ sugar by the tablespoon as necessary to bring the frosting to a thickly spreadable consistency.
Assemble the cake. Place one of the cooled cakes onto a cake stand or other serving platter, and spread about 2/3 of of the buttercream filling into one thick, even layer on top. Place the second cake on top of the frosting and press down evenly on top. Place the remaining buttercream on top of the layer cake and spread across the top and around the sides of the cake in a thin layer to seal in crumbs. Try adding a few lovely fresh blueberries, face out, on the filling layer for decoration.
Make the lemon glaze. In a medium-size bowl, place 2 cups of the confectioners’ sugar and the salt, and whisk to combine well. Add the lemon juice and the butter, and mix to combine. Add more confectioners’ sugar as necessary to create a very thickly pourable glaze. Pour the glaze on top of the layer cake and spread into an even layer on top, allowing it to drip down the sides. Allow the glaze to set either at room temperature or in the refrigerator before slicing and serving.
Originally published on the blog in 2014. Recipe method adjusted slightly; photos and video new.