Preheat oven to 350 degrees F. Line a 12 cup muffin tin with paper liners and set aside.
In a medium bowl combine 1 and 3/4 cups of the flour, baking powder, baking soda, and salt; set aside.
In a separate bowl combine the sour cream, milk, and 2 tablespoons of the lemon juice; whisk well to combine and set aside.
Place the blueberries in a small bowl; add remaining lemon juice and toss to coat the berries; add remaining flour and toss to coat the berries in the flour; set aside.
In a large bowl using a handheld electric mixer, or in the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth and creamy, about 30 seconds.
Add in the granulated sugar and lemon extract (if using) and beat until well combined.
Add in the egg and egg yolk and mix well.
Reduce the mixer speed to low. Add in half of the flour mixture and mix until just combined (this should take about 5 seconds).
Add half of the sour cream mixture and mix until just combined. Repeat with remaining flour and sour cream mix, being sure to mix on low until just combined.
Using a rubber spatula, fold in the floured blueberries (and any excess flour in the bowl) and lemon zest.
Spoon the batter into the prepared muffin cups, filling them 3/4 of the way full.
Bake for 16-20 minutes, or until a toothpick inserted in the center comes out clean.
Allow cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
Once cupcakes are cool, frost and decorate. The cupcakes may also be frozen, unfrosted and stored in a freezer-safe bag, for up to 2 months.
In a medium bowl using a handheld electric mixer, or in the bowl of a stand mixer fitted with the paddle attachment, beat the butter and cream cheese on medium-speed until smooth and creamy.
Reduce the speed to low and gradually add in the powdered sugar and lemon juice, beating until light and fluffy, about 2 minutes.
Add in the vanilla extract, lemon extract (if using) and lemon zest; beat smooth.
If the consistency appears too thick, add a teaspoon of milk or heavy cream, but only if necessary.
The frosting should be creamy and spreadable, but also thick enough to hold its shape. Pipe or spread the frosting on top of each cooled cupcake.
Top each frosted cupcake with a pinch of lemon zest and extra blueberries, optional but pretty!
