Mix dry ingredients: In a medium bowl, stir together flour and salt. Set aside.
Cream butter and sugar: In a large mixing bowl, use an electric mixer to beat the softened butter on medium speed until creamy. Add the powdered sugar and beat on low speed to start, increasing to medium speed, until fluffy.
Mix in egg: Add the egg and beat until combined.
Add flour: Add the flour mixture and beat on low speed until it forms a soft, crumbly dough.
Press dough together: Dump the dough out onto a sheet of plastic wrap. Use your hands to press the dough together into a flattened disc shape. Wrap the dough in plastic wrap and chill in the fridge for at least 2 hours or overnight.
Roll out the dough: On a floured surface, roll out the dough to about ⅛" thick. Use a round scalloped cookie cutter to cut rounds out of the dough.
Push dough into muffin pan: Place a dough round on top of an opening on your muffin pan. Slowly and gently push down on the dough with your knuckle until the dough slips down and fills the hole. Patch any tears with a small piece of excess dough. Repeat with the rest of the dough rounds and the pan is filled.
Chill and preheat: Place the whole pan in the fridge to chill for about 30 minutes while your oven preheats. Preheat the oven to 350°F.
Add baking weights: before baking, place muffin liners in each well of the pan, on top of the dough. Fill each liner with a few baking weights or uncooked rice. Bake for 18-20 minutes or until the edges of the crusts are golden brown.
Let cool: Remove the liners and let the tart shells cool in the pan for about 5 minutes. Remove the tart shells from the pan and transfer them to a wire rack to cool completely.