• Preheat the oven to 350°F. Brush 2 8-inch baking pans with unsalted butter, and dust with gluten free flour. Place a circle of parchment paper at the bottom of each pan, and set aside.
• Whisk together the flour, baking powder, and salt in a medium bowl and set aside.
• In a measuring cup, mix together the milk, lemon juice and vanilla extract, set aside.
• Place the sugar and lemon zest in your mixing bowl, using your hands, rub the mixture between your fingers until the sugar resembles wet sand.
• Add the butter and mascarpone cheese to the sugar, and cream on high speed until it is light and fluffy, scraping down the bowl as needed. Use the paddle attachment for a stand mixer or use a hand mixer for this.
• Add the eggs, one at a time, and mix until well combined.
• Beginning and ending with the flour mixture, add ⅓ of the flour mixture. Mix until just combined, and add ½ of the milk mixture, mixing until just combined. Repeat with another ⅓ of the flour mixture and remaining milk mixture. Finally, add the remaining flour mixture, and mix until just combined, again scraping down the bowl as needed. Gently fold in the berries.
• Divide evenly among the two cake pans. Rest for 10 minutes. Place pans in the oven and bake for 35-40 minutes, or until the cake bounces back when gently pressed in the center or a toothpick inserted in the center comes out clean.
• Remove from the oven, and place on a cooling rack for 30 minutes. Turn the cakes out onto the cooling rack and let cool completely. Wrap tightly in plastic wrap and freeze until ready to use.
• Once thawed enough to slice through the center, cut each cake into two layers. Dollop about 1 cup of Blueberry Frosting on the bottom layer, and spread to within ½ inch of the edge of the cake. Top with a layer of cake, and repeat until all the layers have been topped with frosting. If desired, spread a thin layer of frosting on the outside of the cake.

Blueberry Frosting

• Place blueberries and lemon juice and zest in a small saucepan over medium heat. Stirring occasionally, let the blueberry mixture cook until the blueberries are softened and a syrup has formed, about 12-15 minutes. Remove from heat and let sit until cool.
• Once the blueberry mixture has cooled, add to a mixer fitted with the whisk attachment. Add the butter to the mixer, and whisk on high until the blueberry mixture is well combined with the butter.
• Add the powdered sugar, vanilla extract, and salt to the mixer, and whisk on low until the powdered sugar is well incorporated. Scrape down the bowl, and then turn the mixer on high, whisking until the mixture is light and fluffy, about 10 minutes. Switch the whisk to the paddle attachment, and whip on high for an additional 5 minutes.