Preheat the oven to 350°F / 175°C. Butter and line an 8" square pan with parchment paper.

In a small mixing bowl, toss together the blueberries and 2 tbsp. of flour until all the blueberries are coated. Set aside.

To a medium sized mixing bowl, add the sugar and zest of 2 large lemons. Use your fingers to rub the zest into the sugar.

Add in the flour, baking powder and salt. Use a hand mixer or a stand mixer fitted with the paddle attachment to beat the mixture together until the butter has completely combined with the dry ingredients and no lumps remain.

Add in the oil, egg whites, lemon extract, vanilla, sour cream and milk. Whisk the ingredients together just until a cohesive batter has formed.

Add ⅔ of the blueberries into the batter and gently mix them into the batter.

Pour the cake batter into the prepared cake pan and top it with the remaining blueberries. Bake for 40-45 minutes, or until a toothpick inserted in the center of the cake comes out with only a few moist crumbs.

Prep a small mixing bowl by adding the cubed butter to it, along with a fine mesh sieve. Set aside.

Add the sugar to a small saucepan, then zest the lemon into the pan. Use your fingers to rub the zest into the sugar.

Juice the lemon, and add 28g (2 tbsp.) of lemon juice to the saucepan. Then add in the egg yolks, blueberries and salt.

Set the saucepan over medium heat, stirring constantly (this can be slowly - we just need constant movement happening!) Gently mash down the blueberries as they soften.

Continue to stir the curd until it thickens and reaches a temperature of at least 170°F / 76°C (This took me about 3-5 minutes.)

Remove the curd from the heat and pass it through the fine mesh sieve, over the butter. Mix the curd and the butter together until the butter has melted and the mixture is smooth.

Transfer the curd to the fridge to chill until you're ready to assemble the cake.

In a medium sized mixing bowl with a hand or stand mixer, beat together the mascarpone, sugar, salt and lemon zest just until the mixture is cohesive.

Slowly beat in the heavy whipping cream. Once all of the heavy cream has been added, beat the mixture until stiff peaks form.

Once the cake has fully cooled to room temperature frost it with the mascarpone whipped cream, then swirl in the blueberry curd (you will likely have extra curd.) Garnish with extra blueberries, then slice, serve and enjoy!