Preheat the oven to 350°F (175°C). Position a rack in the center of oven. Line an 8 by 8-inch square baking pan with parchment paper, allowing a few inches overhang on two ends for the easiest removal of blondies.
In a medium bowl, whisk together the almond flour, coconut flour, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, add butter and sweetener. Mix on medium-high until smooth and creamy, about 2 minutes. Add the eggs, one at a time, scraping down sides of the bowl after each addition. Add the vanilla. Mix on medium-high speed until frothy and light colored, 2 minutes. With the mixer on low, slowly incorporate the bowl of dry ingredients until a thick batter forms. Use a rubber spatula to fold in the cacao nibs.
Scrape the batter into the prepared pan, then evenly spread it into the corners and smooth the surface.
Bake until the top is golden brown and the center springs back when lightly pressed with a finger, 20 to 25 minutes.
Let the blondies cool on a wire rack for 20 minutes.
Gently remove the blondies from the pan by pulling up the overhanging parchment paper edges. Cool on the wire rack for another 20 minutes, then peel away the parchment paper and cut into 16 squares.