Preheat the oven to 350°F. Line a 9×9-inch square glass baking dish with parchment paper and set it aside.
Mix together the oil and sugar in a large mixing bowl. Use a hand mixer or stand mixer to combine.
Beat in lemon extract, vanilla extract, lemon zest, and lemon juice. Add one egg at a time and then the egg yolk, mixing well between each one.
Stir together the flour, salt, and baking powder and add to the wet mixture.
Beat together 1 to 2 minutes until fluffy and well combined.
Spread the batter into the prepared baking dish. Bake for 25 minutes. Remove from the oven and allow them to cool for 30 minutes before preparing the glaze.
Whisk together the powdered sugar, water, lemon extract, and lemon juice until a smooth paste-like glaze forms.
Spread glaze over the brownies. Allow brownies to cool completely and the glaze will set so that it’s dry to the touch. Garnish with lemon zest.
Cut into pieces and serve.
Video
Notes
The oil and the egg yolk help to produce an even thicker brownie. The extra egg yolk also gives the brownie a bit more saturated yellow color. You could always add food coloring for an even more vibrant-looking yellow brownie.
Add more or less lemon juice/extract/zest as you desire to adjust the depth of lemon flavor in the brownies and the glaze.
When mixing the glaze, add the liquid ingredients gradually to control the consistency of the glaze. Too much water/liquid will make a very runny and less opaque glaze. I like the white contrast to the brownie color, so I like the glaze to be more opaque and thick. The glaze is very sweet. Just a light glaze or even a drizzle would be great, too.