Preheat the oven to 350F (180C). Line a 7-inch (17.5 cm) square pan, or a 9x5-inch (22.5 x 12.5 cm) loaf pan, with parchment paper; spray or grease exposed sides.
In a large bowl, stir the sweet potatoes, nut butter, milk, syrup, and vanilla until completely blended and smooth.
In a small bowl or cup, stir together the coconut flour, baking powder, cinnamon and salt; add to the sweet potato mixture, stirring until blended. The batter will be thick, but if it seems dry, add a tablespoon or two more milk.
Spread batter evenly in prepared pan, smoothing the top. Sprinkle with pecans, if using.
Bake in the preheated oven for 35 to 40 minutes until puffed and surface of blondies appears dry.
Transfer to a cooling rack and cool completely in pan.
Use the parchment overhang to lift blondies from pan onto a cutting board. Cut into 12 blondies.
Notes
Cooking the sweet potato: Prick the sweet potato(es) (about 1 very large or 2 medium) all over with a fork. Either (1) bake in a preheated 425F oven for 1 hour or until tender or (2) microwave on HIGH for 10 to 15 minutes until tender.Storage: Store in an airtight container in the refrigerator for 1 week or the freezer for up to 6 months.