Follow recipe for basic butter cookies to make dough, then divide dough in half. Divide 1 half (keep remaining dough chilled, covered) into 24 pieces (about 1 1/2 teaspoons each) and form into 1-inch balls. Press each ball evenly onto bottom and up side of an ungreased mini-muffin cup to form a pastry shell (use a piece of plastic wrap to prevent dough from sticking to your fingers). Chill until ready to fill.
Put oven rack in middle position and preheat oven to 375°F.
Whisk together brown sugar, butter, egg, vanilla, and salt until blended, then stir in pine nuts. Fill each pastry shell with about 2 teaspoons filling.
Bake tassies until filling is bubbling and pastry is golden brown, 12 to 15 minutes. Run a sharp thin knife around edge of each pastry, then transfer pan (with cookies) to a rack to cool 10 minutes.
Remove tassies from pans and transfer to rack to cool completely. Make more tassies with remaining dough and filling.
Tassies keep, layered between sheets of wax paper or parchment, in an airtight container at room temperature 1 week.
