Preheat the oven to 350°F (175°C).

In a mixing bowl, combine 1 cup all-purpose flour, 1/2 cup softened unsalted butter, 1/4 cup sugar, and 1/4 teaspoon salt; mix until the mixture resembles coarse crumbs.

Press the dough into the bottom and up the sides of a muffin tin to form cups.

In another bowl, beat 1 large egg, then stir in 1/2 cup packed brown sugar, 1 teaspoon vanilla extract, and 1 cup chopped pecans until combined.

Spoon the filling into each dough cup, filling about three-quarters full.

Bake in the preheated oven for 20 to 25 minutes, or until the edges are golden and the filling is bubbly.

Remove the muffin tin from the oven and let the tassies cool in the tin for 10 minutes.

Carefully remove the tassies from the tin and transfer to a wire cooling rack to cool completely before serving. If desired, dust with powdered sugar.