In a medium bowl combine flour, salt and baking soda. Set a side.
In the bowl of an electric mixer with paddle attachment cream together butter and sugar, until fluffy (for 3-4 minutes).
Add egg and vanilla and mix until well combined. Slowly add flour, salt and baking soda, mixing until just incorporated.
Form a ball from the dough, cover with plastic wrap and chill for 1 hour.
Preheat oven to 350 F (177 C).
Spray a regular 12 cups muffin tin with non stick cooking spray.
Using a 2 tbsp cookie scoop, scoop dough into muffin tin. Press onto bottom to form a well and spread dough to the sides of muffin tin, about half way up.
Bake cookie cups for 10-12 minutes, until edges turn golden.
Remove from oven and using the back of a spoon, carefully press dough down the center of each cookie, to form a cup (this dough usually spreads while baking). Let them cool in the muffin tin for 5-10 minutes, then tip cookie cups onto a clean towel. Let them cool completely.