Preheat oven to 350 degrees. Spread pecans in a single layer on a baking sheet; toast until crisp, about 7 minutes. Let cool, then coarsely chop 1/2 cup pecans. Set remaining aside.
Place 2 tablespoons butter in a 9-inch round cake pan; heat in the oven until melted, about 3 minutes. Remove from oven; sprinkle 1 tablespoon sugar over butter. Scatter chopped nuts over sugar; set aside.
In a food processor, process reserved pecans and flour until nuts are very finely ground. Set aside. With an electric mixer, beat remaining 1/2 cup butter and remaining cup sugar until light and fluffy. Beat in eggs, one at a time, until combined.
In a small bowl, combine flour mixture, baking powder, and salt; fold into egg mixture. Stir in zest. Pour into prepared pan, smoothing top.
Bake until a toothpick inserted in center comes out with a few moist crumbs, 35 to 40 minutes. Cool 10 minutes in pan; invert onto a rack to cool completely, bottom side up so the butter-pecan layer is on top.