Peel cooked sweet potato, add flesh to a mini food processor and process until smooth. (You can also mash by hand using a potato masher. Make sure you mash it until completely smooth.)
Add 1 cup sweet potato puree (222g), remaining ¼ cup flour (31g), 1 teaspoon baking powder, ½ teaspoon baking soda and ½ teaspoon kosher salt to batter; mix to combine.
Transfer batter to prepared pan, smooth top and sprinkle streusel evenly over top.
Bake cake until puffed and golden, and a toothpick inserted in center comes out clean, about 40 minutes. Cook cake in pan, on wire rack, for 15 minutes. Carefully remove cake and allow to cool completely on wire rack.