Preheat oven to 350 degrees. Use parchment paper or a silpat mat to line a baking sheet.
Cream the butter, brown sugar, and granulated sugar in a stand mixer until light and fluffy (about 2-3 minutes). Scrape the sides of the bowl to make sure all ingredients are blended.
Add in the egg, lemon zest, lemon juice, and lemon extract and beat until combined.
Gently mix in flour, baking soda, and salt. Mix until all combined. Be careful not to overmix the dough.
Scoop out about 1 tablespoon (or use a 1 tbsp cookie scoop) and roll into a dough ball. Repeat with rest of dough and place dough balls on a wax covered plate and freeze for 15 minutes.
Remove the dough from the freezer and bake at 350 degrees on the prepared baking sheet for 9 minutes. Cool cookies cookie sheet for 10 minutes before removing to cooling rack.
Melt the butter, powdered sugar, lemon juice and lemon zest in a small pan. Stir until the sugar dissolves completely.
Using a pastry brush, glaze each cookie with the glaze and top with a lemon slice. (NOTE: You can definitely leave the lemon slice off.)