Heat oven to 350F.
Prepare an 8×8” baking pan by greasing the sides well.
In a large bowl, whisk melted butter and sugar for 1-2 minutes.
Add the 1 ⅓ Cup flour, baking powder and salt, mixing well.
Whisk in ½ the lemon zest, lemon juice, the egg plus one egg yolk, and vanilla. Mix until all ingredients are well combined.
Toss the blueberries in 1 tsp flour until they are well-coated.
Gently fold the blueberries into the batter.
Pour batter into the baking pan.
Bake for 20-25 minutes until a toothpick inserted into the center (but not through a blueberry) comes out clean. If it starts to brown around the sides or top, cover with foil until baked through.
Place the pan on a wire rack to cool.
Frosting:
Whisk together all ingredients until smooth.
When the cake is completely cool, spread frosting evenly over the top.
Optional: Dot with blueberries.
