Whisk dry ingredients (flour and cornstarch) together and set aside.
Cream butter and sugars with paddle attachment, on medium-low speed, until fluffy and pale in color (about 5 minutes).
Add egg and extract(s) and mix on low speed until thoroughly combined. Scrape down the paddle with a spatula, and then continue to mix on low speed for another 30 seconds to ensure everything is fully incorporated.
Add dry ingredients, all at once, and mix on low speed just until dough starts to come together and no dry bits remain. Do not overmix.
Turn dough out onto cling wrap, tightly wrap, and refrigerate overnight (or for at least 4 hours).
Knead, roll out ¼" thick using flour, cut out 3" circles of cookie dough, and gently press dough circles into sides and bottoms of tart pans.
Use a tart tamper, shot glass, or condiment ramekin to press dough flat into bottom of tart pans. Remove excess dough from tart pans with knife or by gently pressing with the palm of your hand.
Gently press dough against sides of tart pans so that it rises about ⅛" above the top of the tart pan.
Freeze filled tart pans until dough is firm before baking.
Preheat oven to 350°F
Par-bake tartlet shells for 12-14 minutes, until set.
Immediately after removing shells from oven, use your tart tamper, shot glass, or condiment ramekin to tamp down inside of tart shell while still hot.
Leave shells in their pans.