Position a rack in the center of the oven and preheat to 350º. Line an 8x8-inch square pan with parchment paper on the bottom and all sides.

Spread the pecans on a small baking sheet and toast in the oven until golden and fragrant, 10 – 12 minutes. Remove and let cool, then break or chop into coarse pieces.

Place the brown sugar in a large bowl and stir in the warm melted butter, then whisk in the egg and vanilla. Place a strainer over the bowl and sift in the sweet rice, chestnut, and tapioca flours with the baking powder and salt. Stir to combine, then stir vigorously for a few seconds; this will help the blondies hold together better when baked. Stir in the nuts, dark and white chocolates, and cranberries. Scrape the dough into the prepared pan and spread evenly; it will be a fairly thin layer.

Bake the blondies until puffed, golden, shiny and slightly cracked on top, and a toothpick inserted comes out clean or with a few crumbs, 20-30 minutes. Remove to a wire rack and let cool to room temperature (or chill until firm, 1 hour, for cleaner cuts). Use the parchment handles to remove the blondies from the pan and onto a wire rack. Drizzle with the melted chocolate. Once the chocolate has set, cut into 16 squares.

The blondies are best the day they are made, but will keep, in an airtight container at cool room temperature or (better yet) refrigerated, for several days.