Preheat the oven to 350°F. Grease a 24-cup mini muffin pan.
In a mixing bowl, beat the softened butter and cream cheese until well combined.
Add the flour and mix until a soft dough forms.
Roll the dough into 1-inch balls. Place one ball into each prepared mini muffin cup.
Using your thumb or a tart tamper, press each dough ball into the bottom and up the sides of the muffin cup to form a small pastry shell.
In a separate bowl, combine the melted butter, brown sugar, egg, vanilla extract, and pecans. Mix until well combined.
Spoon the pecan filling into each pastry shell, filling just to the top of the shell. Avoid overfilling.
Bake for 20–25 minutes, or until the pastry is golden brown.
Let the pecan tassies cool in the pan for a few minutes.
Carefully run a thin knife around the edges to loosen any filling that has stuck to the pan. Gently remove the tassies, inverting the pan if needed, and allow them to finish cooling.