Cook the blueberry compote. Combine the 400 g blueberries, 110 g granulated sugar, lemon juice, and 1 g salt in a medium heavy-bottomed saucepan. Cook over medium heat for 5 to 7 minutes, stirring and lightly crushing about half the berries, until juicy and simmering. Whisk the cornstarch with the cold water, stir it into the fruit, and boil for 60 to 90 seconds until glossy and thick enough to leave a trail behind the spatula. Transfer to a shallow container, cover the surface, and refrigerate until completely cold.
Prepare the pans and preheat the oven. Preheat the oven to 175°C (350°F). Lightly grease three 8-inch round cake pans, line the bottoms with parchment rounds, and grease the parchment. Leave the sides only lightly coated. Weigh the ingredients, bring the eggs and buttermilk to room temperature, and make sure the cake-layer butter is pliable but not greasy.
Combine the dry and liquid ingredients. Reserve 15 g of the cake flour for coating the blueberries. Place the remaining 330 g cake flour, 350 g granulated sugar, baking powder, baking soda, and 5 g salt in a stand mixer bowl. Mix with the paddle attachment on low speed for 30 seconds. In a separate bowl, whisk the eggs, buttermilk, vanilla, and lemon zest until combined.
Reverse-cream the cake batter. Add the softened 170 g butter to the dry ingredients a tablespoon at a time while mixing on low. Continue mixing for approximately 2 minutes, scraping the bowl once, until the mixture resembles evenly damp sand without large butter pieces. Slowly add the oil and mix for 30 seconds. Add half the egg and buttermilk mixture, beat on medium speed for 60 seconds, and scrape the bowl. Add the remaining liquid in two additions on low speed, mixing just until smooth after each addition.
Fold in the blueberries and divide the batter. Toss the 250 g blueberries with the reserved 15 g cake flour. Keep frozen blueberries frozen until adding them to the batter. Gently fold the berries into the batter using a flexible spatula until evenly distributed. Divide the batter among the three prepared pans, approximately 570 g per pan, and smooth the surfaces. Tap each pan lightly on the counter to release large air pockets.
Bake the cake layers. Bake at 175°C (350°F) for approximately 24 to 29 minutes. Position the three pans on the center rack only if they fit comfortably with adequate space for air circulation. Otherwise, use two racks and switch the pans' positions after approximately 18 minutes. Begin checking at 24 minutes and allow additional time if necessary. The tops should spring back gently, the edges should begin pulling away from the pans, and a skewer inserted between the blueberries should come out with moist crumbs but no wet batter. The centers should register approximately 96–98°C (205–208°F). Avoid testing directly through a blueberry.
Cool and release the cake layers. Cool the cakes in their pans on wire racks for 15 minutes. Carefully loosen the edges with a thin offset spatula, invert onto the racks, remove the parchment, and turn the layers upright. Allow them to cool completely, approximately 90 minutes. For easier leveling and stacking, wrap the cooled layers and refrigerate for 30 minutes. Never assemble the cake while the layers are warm.
Prepare the mascarpone frosting. Beat the softened 340 g butter with the paddle attachment on medium speed for 2 to 3 minutes until smooth and slightly lightened. Add the sifted confectioners' sugar and 2 g salt in three additions on low speed, then beat for 1 minute. Add the cold mascarpone in four portions, mixing on low just until incorporated after each addition. Mix in the vanilla for approximately 15 seconds. Stop when the frosting is smooth, thick, and holds a soft peak. Avoid overmixing, which can cause mascarpone frosting to become loose or grainy.
Fill and stack the cake. Level the chilled cake layers with a serrated knife if necessary. Secure the first layer to an 8-inch cake board with a small amount of frosting. Spread a thin layer of frosting over the cake, then pipe a continuous 1/2-inch-high frosting dam around the perimeter. Add approximately 170 g completely chilled blueberry compote inside the dam and spread evenly. Repeat with the second layer, using another 170 g compote. Reserve up to 80 g for the topping. If the compote yields less than 420 g, prioritize the filling layers and use only the remainder for decoration. Place the third cake layer upside down for a level top, gently settle it into position, and check that the stack is straight.
Crumb-coat, frost, and decorate. Apply a thin crumb coat over the top and sides, filling gaps between the layers. Refrigerate for approximately 30 minutes until firm. Spread the remaining mascarpone frosting evenly over the cake and smooth with a bench scraper. Work promptly to prevent the frosting from softening. If it begins losing definition, refrigerate the cake for 15 to 20 minutes before continuing. Pipe a frosting border around the top, spoon up to 80 g reserved cold blueberry compote inside, and arrange the 150 g fresh blueberries over it. Refrigerate for at least 30 minutes before slicing. Cut into 16 portions using a sharp knife warmed in hot water and wiped completely dry, cleaning between cuts.
