Preheat the oven to 350°F/180°C and line 2 half size baking sheets with parchment paper or silicone mats.
In a large bowl, combine lemon cake mix, eggs, and oil. With a spatula, mix together until combined. A few lumps are ok, do not over mix. For thicker cookies, add half a cup of flour. You can add up to 1 cup of flour, depending on the texture you'd like. (see note 1)
Using a small OXO cookie scoop (1 ½ tbsp size), scoop out cookie dough balls and place 3 inches apart on the baking sheets.
Bake in a preheated oven for 12 minutes, then cool down completely on a wire rack.
To make the glaze, in a bowl combine the powdered sugar, lemon zest and 1 tablespoon of lemon juice. Add additional lemon juice to create a thick glaze.
When the cookies have cooled down completely, glaze the cookies while they are still on the wire rack, so that the glaze can drip off of the cookies if needed. Leave to set until dry.
Notes
Adding flour is optional, and should only be added if the cookie dough seems very thin. Different brands of cake mix will give slightly different results.
Be sure to allow 3 inches of space around each cookie when baking. These cookies will spread quite a bit.
Don't try to glaze cookies while they're still warm. Allow them to cool completely before making and adding the glaze.
To Store: Keep these cookies in a single layer in an airtight container for 2-3 days. Lemon cookies can be frozen without the glaze for up to 2 months.