Preheat the oven to 300F. In a large bowl, whisk together the sunflower seed flour, cocoa powder, sweetener, baking powder, and salt. Stir in the egg, butter, and vanilla until the dough comes together. Transfer the dough to a large silicone baking mat or a large piece of parchment paper. Top with another piece of parchment and roll out as thinly as possible (without being see-through!). Remove the top piece of parchment. Use a sharp knife or a pizza cutter to score into small squares, about 3/4 inch wide. Bake 25 to 30 minutes, until firm to the touch. Remove and let cool completely. If it’s not completely crisp, return to a warm oven (175F) for another 20 or 30 minutes. Break into pieces and serve.