In a large bowl, mix butter with powdered sugar and salt until light and fluffy.
Add egg and continue mixing until well combined and creamy.
Incorporate the flour into the butter mixture until combined. Make sure you don’t overwork the dough.
Wrap the dough with plastic wrap.
Form into a disc and refrigerate for at least 30 minutes.
Flour the working surface, and roll the dough between two pieces of parchment paper or plastic wrap until it's about l ⅛ (3 mm) thick.
Use a 3-inch (7,5 cm) round cookie cutter and cut circles, re-rolling scraps as needed.
Carefully press the dough into mini tart molds. I used 2 1/3-inch - 6 cm tart molds. Freeze for 15 minutes before baking.
Meanwhile, preheat the oven to 350°F (180°C).
Prick the bottom of the tarts with a fork.
Bake for about 13-15 minutes or until slightly golden at the edges.
Let them cool completely.