Position a rack in the upper third of the oven and preheat to 325ºF. Line the bottom and sides of an 8-inch-square baking pan with parchment paper.
Place the pecans in single layer on a rimmed baking sheet and toast in the oven until lightly browned and fragrant, 8 to 10 minutes. Cool, coarsely chop, and reserve.
Raise the oven temperature to 350ºF. Place the brown sugar in a large bowl and stir in the butter. Let the mixture cool to warm, 5 minutes, then stir in the egg and vanilla until combined. Set
a strainer over the bowl and sift in the sweet rice flour, coconut flour, baking powder, and fine sea salt. Stir vigorously until well combined, then stir in the pecans and chocolate. Scrape the dough into the prepared pan and spread it evenly into a thin layer. Sprinkle the top with the flaky sea salt.
Bake until slightly puffed on top and a toothpick inserted near the center comes out clean or with a few moist crumbs, 18 to 22 minutes. Let cool completely, remove from the pan, and peel away the parchment. For uniform squares, trim the edges with a large knife. Cut into 16 bars and serve.
Extras can be stored in an airtight container at room temperature for 3 days.