1 (11 ounce) package pie crust mix (such as Betty Crocker®)
3 tablespoons cold butter
⅓ cup ice cold water
4 egg yolks
⅓ cup white sugar
or to taste
⅓ cup heavy cream
⅓ cup milk
3 drops vanilla extract
Preheat oven to 400 degrees F (200 degrees C). Spray 12 muffin cups with cooking spray.
Place the pie crust mix into a bowl, and lightly rub the butter and water into the mix until the mixture holds together well enough to form a ball. Turn the dough out onto a floured surface, and knead several times to make the dough more elastic. Form into a ball again, and cut the ball in half; cut each half into half again (4 pieces); cut each quarter into thirds to make 12 total pieces. Roll the pieces into balls, and set aside.
Beat together the egg yolks, sugar, cream, milk, and vanilla extract in a bowl until light and foamy, about 3 minutes; strain the filling through a fine mesh strainer into a clean bowl.
Press a dough ball into the bottom and about 1 1/2 inches up the sides of a prepared muffin cup; repeat for remaining dough pieces. Spoon each crust about 80 percent full of the egg yolk mixture.
Bake in the preheated oven until the crusts are golden brown, the filling is set, and the tarts are lightly browned, 15 to 20 minutes.