Preheat the oven to 350°F. Grease an 8-by-8-by-2-in pan with nonstick cooking spray or softened butter, line with parchment paper, and grease again.
In a medium bowl, add the flour and salt, and whisk to combine. Place the butter, zest, orange extract and tequila in the bowl of a stand mixer fitted with the paddle attachment, and mix on medium speed until soft and light, about 3 minutes. Add both sugars and continue mixing for 2 more minutes or until fluffy, scraping down the bowl as needed.
With the mixer on low, slowly add the dry ingredients and mix just until incorporated. Remove the bowl from the mixer and continue mixing with a rubber spatula until fully combined. Transfer to the prepared pan. The dough will be quite sticky. Drape a piece of plastic wrap over the dough, and using your hands, press the dough into the pan.
Bake for 33 to 36 minutes rotating the pan at the halfway point. The cookies are done when they are just slightly browned and dry to the touch. I like to press down the edges of the shortbread while still in the pan with a dish towel to flatten them if they have puffed up while baking. Immediately slice the dough with a paring knife by making 1 horizontal line that divides the dough in half, and 6 vertical lines from top to bottom. With the tines of a fork, poke each cookie 5 times. Let cool to room temperature before glazing. You can speed this up by placing the pan in the freezer for about 45 minutes.
To make the glaze, combine all the ingredients in a small bowl and whisk to combine. if the glaze is too thick add a teaspoon more of water. Remove the cooled cookies from the pan, separate them, and decoratively dribble the glaze on top of the cookies. Sprinkle with the flaky sea salt, if using. Let set before serving. The cookies will keep in an airtight container on the counter for up to 3 days.