Use a stand mixer with the paddle attachment or a handheld electric mixer on medium speed to beat the butter and cream cheese until smooth. Turn the mixer to low and add the flour in several increments. Beat until the dough pulls into a ball. Cover the bowl with plastic wrap and refrigerate for 1 hour.

Preheat oven to 350 degrees and spray a mini muffin pan generously with baking spray.

Roll the dough into about 3/4-inch balls. Gently press the balls into and up the sides of the prepared pan. Sprinkle about a teaspoon of chopped pecans into each tart hole.

Lightly beat the egg in a medium bowl. Add the brown sugar, vanilla extract, melted butter, and salt to the bowl with the egg and stir to combine. Spoon the mixture evenly into the tarts and sprinkle with the remaining chopped pecans.

Bake for 20-23 minutes or until the filling is puffy and the crust is lightly browned. Let them cool in the pan for about 15 minutes before using a knife to run the rim of each tassie, and gently remove them to a wire rack to cool.