Add the vegetable oil and granulated sugar to a mixing bowl and beat until well combined.

Add the eggs and mix until smooth.

Add the sour cream and mix until incorporated.

Add the lemon zest, lemon juice, lemon extract, and buttermilk, then mix until well combined.

In a separate bowl, whisk together the all-purpose flour, baking powder, and salt.

Gradually add the dry ingredients to the wet mixture and mix until just combined.

Add the blueberries and gently fold them into the batter.

Add the blueberry purée and gently swirl it through the batter.

Transfer the batter to a prepared cake pan and spread it evenly.

Bake at 350°F (175°C) for 35–45 minutes, or until a toothpick inserted into the center comes out clean.

Remove from the oven and allow the cake to cool completely.

Cut the cake into pieces.

Prepare the frosting by beating the cold mascarpone cheese and heavy whipping cream, then add the powdered sugar, vanilla extract, lemon curd, and blueberry purée and beat until smooth, thick, and fluffy.

Spread the lemon blueberry frosting over the cake pieces.

Serve and enjoy.