Beat the brown sugar and butter with an electric mixer in a large bowl until smooth. Beat in the egg and vanilla extract until completely incorporated. Mix the flour, baking powder, and salt together in a separate bowl, then stir the flour mixture into the butter mixture until just incorporated. Fold in the pecans mixing just enough to evenly combine.
Roll the dough into a log, and wrap in wax paper. Chill the dough in the refrigerator for at least one hour.
Preheat an oven to 350 degrees F (175 degrees C). Slice the dough and arrange the cookies on a baking sheet.
Bake in the preheated oven until the edges are golden, about 12 minutes. Allow the cookies to cool on the baking sheet for 1 minute before removing to a wire rack to cool completely.