Preheat oven to 350ºF. 

Lightly grease a 9×13-inch baking pan.

In a bowl add the blueberries and 2 teaspoons of flour and toss to combine. 

In another bowl combine the remaining flour, cornstarch, baking powder, baking soda and salt. 

In the bowl of a stand mixer fitted with the paddle attachment beat the eggs and sugar. 

Add the vanilla extract, lemon juice, lemon zest, buttermilk, melted butter and oil and beat until combined. 

Add the flour mixture and beat until incorporated. 

Fold in the blueberries. 

Spread the batter into the prepared pan.

Bake in preheated oven at 350 ºF for 35-40 minutes or until a toothpick inserted in the center of a muffin comes out clean.

Remove from the oven and allow to cool completely. 

To make the frosting, in the bowl of a stand mixer fitted with the paddle attachment beat the softened butter, confectioners sugar and lemon juice, scraping bowl occasionally, until smooth and creamy about 3-5 minutes. 

Spread the frosting over the cooled cake.