Part 1: Bake the Lemon Blueberry Cake Layers

1. 1. Preheat Oven and Prepare Pans: Preheat your oven to 350°F (175°C). Grease your chosen cake pans thoroughly. Line the bottoms with parchment paper circles, then grease the parchment. Lightly flour the pans or use baking spray with flour.

1. 1. Combine Dry Ingredients: In a medium bowl, whisk together the sifted cake flour, baking powder, baking soda, and salt. Set aside.

1. 1. Cream Butter and Sugar: In the bowl of a stand mixer fitted with the paddle attachment (or using a large bowl and a handheld mixer), beat the softened unsalted butter on medium speed until creamy, about 1 minute. Gradually add the granulated sugar and continue to beat on medium-high speed for 3-4 minutes, until light, fluffy, and pale.

1. 1. Add Eggs and Mascarpone: Add the room temperature eggs one at a time, beating well after each addition until fully incorporated. Scrape down the sides of the bowl. Beat in the room temperature mascarpone cheese until just combined (do not overbeat).

1. 1. Add Lemon and Vanilla: Beat in the lemon zest, fresh lemon juice, and pure vanilla extract until combined. The batter might look slightly curdled at this stage; this is okay.

1. 1. Alternate Dry and Buttermilk: Reduce the mixer speed to low. Add the dry ingredient mixture in three additions, alternating with the buttermilk in two additions, beginning and ending with the dry ingredients. (Dry -> Buttermilk -> Dry -> Buttermilk -> Dry). Mix on low speed only until just combined after each addition.

1. 1. Fold in Blueberries: In a small bowl, gently toss the fresh (or frozen, unthawed) blueberries with the 1 tablespoon of all-purpose flour. This helps prevent them from sinking. Gently fold the floured blueberries into the cake batter using a spatula, being careful not to overmix.

1. 1. Divide Batter and Bake: Evenly divide the batter between the prepared cake pans. Smooth the tops.

1. 1. Bake for 30-40 minutes (8-inch pans may take slightly less time than 9-inch), or until a wooden skewer inserted into the center comes out clean or with a few moist crumbs (avoid hitting a blueberry). The cakes should be golden and spring back when lightly touched.

1. 1. Cool Cakes: Let the cakes cool in their pans on a wire rack for 15-20 minutes. Then, carefully invert them onto the wire rack, remove the parchment paper, and let them cool completely to room temperature before frosting (at least 2 hours).

Part 2: Make the Lemon Mascarpone Frosting

1. 1. Combine Ingredients (Except Optional Heavy Cream): In a large, chilled bowl (if possible), combine the COLD mascarpone cheese, sifted powdered sugar, fresh lemon juice, lemon zest, pure vanilla extract, and pinch of salt.

1. 1. Mix Gently: Using an electric mixer (preferably handheld for more control, or stand mixer with whisk attachment) on LOW speed, beat the ingredients together just until they are combined and smooth. This should only take about 30 seconds to 1 minute. Be very careful not to overbeat mascarpone on its own, as it can curdle or become runny.

1. 1. Optional – For Lighter Frosting with Heavy Cream: If you want a lighter, more voluminous, and slightly more stable frosting: In a separate, clean, chilled bowl, whip the COLD heavy cream with an electric mixer until stiff peaks form. Gently FOLD the whipped cream into the mascarpone mixture in two or three additions until just combined and no streaks remain. Again, do not overmix.

1. 1. Taste and Chill: Taste the frosting. If it needs more sweetness, you can gently fold in a little more sifted powdered sugar. If it needs more tang, a tiny bit more lemon juice (be cautious as too much liquid can make it runny). If the frosting seems too soft, cover and refrigerate it for 15-30 minutes to firm up slightly before using. Mascarpone frosting is best used relatively soon after making.

Part 3: Assemble and Frost the Blueberry Lemon Mascarpone Cake

1. 1. Level Cakes (If Necessary): If your cooled cake layers have domed tops, use a long serrated knife to carefully level them.

1. 1. First Layer: Place one cake layer (bottom side up, if leveled) onto your cake stand or serving plate.

1. 1. Fill: Spread about one-third of the lemon mascarpone frosting evenly over the top of the first cake layer.

1. 1. Second Layer: Carefully place the second cake layer on top. Press down gently.

1. 1. Frost Top and Sides: Spread the remaining frosting over the top and sides of the cake. Mascarpone frosting is often best applied with a slightly more rustic or swirled finish rather than aiming for perfectly sharp, smooth edges like a traditional buttercream, as it can be softer. An offset spatula is great for this.

1. 1. Garnish (Optional): Decorate the top of the cake with fresh blueberries, thin lemon slices or twists, and/or fresh mint sprigs. A light dusting of powdered sugar can also be pretty.

1. 1. Chill Before Serving (Important): Refrigerate the frosted cake for at least 1-2 hours (or up to 4-6 hours) before serving. This allows the frosting to set properly and the flavors to meld. It also makes for cleaner slices.

1. 1. Serve: Let the cake sit at room temperature for about 15-20 minutes before slicing for the best texture (mascarpone frosting is best when not ice-cold). Use a sharp, thin-bladed knife, wiping it clean between slices.