If you like, once you have zested the orange for the batter, you could cut the fruit into sections to serve alongside the moist and crunchy cake.
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.
