Preheat oven to 350 degrees.
Add sweet potatoes, eggs, buttermilk, melted butter and brown sugar to a large bowl. Mix together until combined. Add flour, baking powder, salt, and pumpkin pie spice. Stir together until smooth and all combined.
Pour into a greased 2 quart baking dish or 9 x 13- inch pan.
In a small bowl, add all streusel ingredients and mix together until a fine crumble.
Sprinkle over cake batter.
Bake on center oven rack for 1 hour 15 minutes - 1 hour 20 minutes or until a toothpick inserted comes out mostly clean.
Allow to cool to room temperature before serving.