Put the lemon zest and sugar in your mixing bowl and mix them with a fork or rub them with your fingertips.
Add butter and cream the mixture until it is light and fluffy with a hand-held mixer or with paddle attachment if using a stand-mixer.
Mix in the egg.
Add lemon juice and mix until combined.
Finally, on the low speed, add in flour gradually and mix until incorporated.
Divide the dough in half and put each on the plastic film. Make approximately 6 inches (15 cm) long, 2 inches (5cm) thick logs out of each dough and keep them in the freezer for 40 minutes, or 4-5 hours in the fridge until they are firm.
Preheat the oven to 350°F(177C°). Line baking sheet with parchment paper or silicone baking mat. Set aside.
Remove the doughs from the freezer and cut them into ⅕ inch(5mm) thick slices and bake for 7-9 minutes or until they slightly change color. Turn off the oven and keep them for an extra 5-6 minutes or until the edges are lightly golden brown. Remove the cookies from the oven and let them cool for 5 minutes on the baking sheet before transferring them to a wire rack to completely cool.