Spread pecans on a rimmed baking pan, and toast in the oven until golden -- about 10 minutes. Set aside to cool.
Reduce temperature to 350*. Sift flour, baking powder, and salt in a medium bowl. Set aside.
In the bowl, of an electric mixer fitted with the paddle attachment, beat butter and 1 1/2 cups light brown sugar on medium speed until light and fluffy., about 2 minutes. Add egg and vanilla, beat on low speed until combined.
Drop batter by level tablespoons onto baking sheets, about 2" apart.
Bake until firm and barely golden, 10-12 minutes, rotating halfway through.
Transfer to a wire rack to cool 5 minutes on baking sheets.
Remove cookies from baking sheet and transfer to wire rack until completely cool.In a small saucepan, combine remaining 1 1/4 cups light brown sugar and cream and bring to a full boil over medium heat. Cook, stirring constantly, about 1 minute. Remove from heat. Working quickly, add confectioners sugar and whisk until smooth. Add pecan pieces and stir until slightly thickened. If mixture thickens, return to heat and add cream 1 tsp at a time. Spoon tsp on each cookie and set aside until firm.