1. Prepare the Lemon Curd:

1. In a medium saucepan, whisk together the granulated sugar and eggs until well combined.
2. Stir in the lemon juice, melted butter, and lemon zest.
3. Cook over medium heat, stirring constantly, until the mixture thickens and coats the back of a spoon (about 7–10 minutes).
4. Remove from heat and strain through a fine-mesh sieve into a bowl to remove any lumps.
5. Cover with plastic wrap, pressing it directly onto the surface, and refrigerate until completely cooled.

2. Bake the Cake Layers:

1. Preheat the oven to 350°F (175°C). Grease and flour three 8-inch round cake pans and line the bottoms with parchment paper.
2. In a large mixing bowl, combine granulated sugar, vegetable oil, eggs, lemon zest, and lemon extract. Beat until well combined.
3. Mix in the light sour cream until smooth.
4. In a separate bowl, whisk together flour, baking powder, and salt.
5. Gradually add the dry ingredients to the wet mixture, alternating with the lemon juice and buttermilk, beginning and ending with the dry ingredients. Mix until just combined.
6. Toss the blueberries with 2 tablespoons of flour to prevent them from sinking, then gently fold them into the batter.
7. Divide the batter evenly among the prepared pans and smooth the tops.
8. Bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean.
9. Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

3. Prepare the Lemon Mascarpone Whipped Cream:

1. In a large mixing bowl, beat the cold mascarpone cheese on medium speed until smooth.
2. Add the cold heavy whipping cream, powdered sugar, lemon extract, and lemon zest.
3. Beat on low speed until combined, then increase to high speed and whip until stiff peaks form. Be careful not to overmix.

4. Assemble the Cake:

1. If necessary, level the tops of the cooled cakes with a serrated knife for even layers.
2. Place one cake layer on a serving plate or cake stand.
3. Spread a thin layer of lemon curd over the top, followed by a layer of mascarpone whipped cream.
4. Repeat with the second cake layer, adding lemon curd and mascarpone whipped cream.
5. Place the final cake layer on top and spread the remaining mascarpone whipped cream over the top and sides for a smooth finish.
6. Garnish with fresh blueberries and lemon zest, if desired.
7. Refrigerate for at least 1 hour before serving to allow the flavors to meld.