Set the oven to 325°F (163°C) and coat a 24-cup mini muffin pan with nonstick cooking spray. Cover the bottoms and sides of the wells so the pastry has an evenly greased surface.
Using a paddle-equipped stand mixer or handheld mixer, blend the softened butter and cream cheese for 30 seconds to 1 minute, then mix in the flour and salt until incorporated. The creamy mixture should lose its visible lumps before the flour goes in, and the finished dough should look cohesive.
Place about 2 teaspoons or 13 grams of dough in each well, then shape it across the bottom and up the sides with your fingers or a tart tamper. Aim for an even shell without a bulky bottom or a paper-thin rim.
Refrigerate the pan of shaped pastry cups while you make the filling. The chilled dough should feel firmer and less soft than it did during shaping.
Combine the brown sugar, melted butter, egg, and vanilla with a gentle whisking motion, stopping once blended, then fold in the chopped pecans. The filling should look evenly mixed rather than foamy.
Take the pan out of the refrigerator and spoon approximately 1 heaping teaspoon of filling into each pastry shell, dividing it among all 24 cups. Keep the mixture contained within the pastry rather than running onto the metal rims.
Bake the filled cups at 325°F (163°C) for 20 to 25 minutes. Look for lightly golden brown tops as the baking cue.
Rest the baked tassies in their pan for about 5 minutes, then carefully work a butter knife around each shell, lift it out, and place it on a wire rack to cool completely. Handle the pastry gently because the warm shells are still delicate.