Preheat the oven to 350°F. Line an 8x8-inch square baking pan with a parchment paper sling.
First, add the butter, eggs, egg yolk, lemon juice, lemon zest, vanilla extract, and sugar to a medium bowl or large bowl.
Whisk the ingredients together until combined.
Next, add the flour, cornstarch, baking powder, and salt.
Stir just until combined.
Now is the time to include any optional add-ins in the brownies, such as white chocolate chips.
Pour the mixture into the prepared baking dish and place in the oven and bake for 18–20 minutes. Remove from the oven and allow the brownies to cool for about a half hour.
While the brownies are cooling, create the glaze by combining the ingredients in a small bowl until smooth. Glaze the top of the brownies once they've cooled, then refrigerate for 1-2 hours until the glaze is set and the bars are chilled.