Preheat oven to 350°F. Line a 9-by-13-inch baking dish with parchment.
Place sweet potatoes in a steamer basket set over a pan of briskly simmering water; steam until completely soft, 18 to 20 minutes. Remove from steamer basket and transfer to a food processor. Add dates and process until smooth.
In a large bowl, combine almonds, buckwheat flour, cacao nibs, cacao powder, honey, orange zest and cinnamon. Add sweet potato mixture; fold everything together. Transfer mixture to prepared baking dish, spreading evenly. Bake for about 20 minutes. Let cool in baking dish on a wire rack for at least 20 minutes. Dust with cacao powder, if desired, then cut into pieces and serve.