Preheat the oven to 350 F. Spray an 8x8 or 9x9 pan with non-stick baking spray. Line with parchment paper and a sling if you want to easily be able to remove the bars from the pan for serving.
In a large microwave-safe bowl, melt the butter and sugar for 1-1.5 minutes, then whisk to combine. The butter should be fully melted, the sugar starting to dissolve, and the mixture warm.
½ cup butter, ¾ cup sugar
Add the lemon juice, zest, eggs and yolk, and vanilla. Whisk vigorously for 30-40 seconds.
3 tablespoon lemon juice, 1 tablespoon lemon zest, 2 eggs + 1 yolk, 1 teaspoon vanilla
Add the dry ingredients and stir with a rubber spatula until just combined. Don’t overmix.
1 cup flour, ¼ teaspoon salt, ¼ teaspoon baking powder
Pour into the prepared pan and bake for 20-25 minutes, until a toothpick comes out mostly clean. Cool completely.
While cooling, combine the glaze ingredients in a small bowl with a whisk. Once the lemonies are cool, spread with the glaze. Slice and serve.
½ tablespoon butter softened, ½ cup powdered sugar, 1-2 tablespoon heavy cream or half n half, ½ tablespoon lemon zest about 1 large lemon, ½ tablespoon lemon juice about 1 large lemon