Preheat your oven to 350°F (convection). Line an 8×8 or 9×9 inch pan with parchment paper, leaving an overhang for easy removal later.

In a stand mixer or using a hand mixer, beat the room-temperature butter until light and slightly off-white. This creates a soft and fluffy base for your cookie bars.

Add the granulated sugar to the butter and beat until the mixture becomes light and airy.

Add the egg and mix until fully combined. Stir in the lemon extract, making sure the flavor is evenly incorporated.

In a separate bowl, whisk together the flour, baking powder, and baking soda. Gradually add the dry ingredients to the wet mixture, mixing on low speed or folding gently by hand until just combined. Avoid overmixing to keep the bars tender.

Spread the cookie dough evenly in the prepared pan, smoothing the top with a spatula. Bake until the edges are lightly golden and a toothpick inserted in the center comes out mostly clean. Let the bars cool completely in the pan before frosting.

In a medium bowl, beat the room-temperature butter until light and off-white. Gradually add powdered sugar and mix until fully incorporated. Add lemon extract, lemon zest, and heavy cream, then whip until the frosting is light and fluffy.

Once the cookie bars are fully cooled, spread the lemon frosting evenly over the top with a spatula.

Use the parchment overhang to lift the bars from the pan. Cut into squares and enjoy! Store in an airtight container at room temperature or in the fridge if you prefer a firmer frosting.