Pre-heat the oven to 350°F.
In a springform pan, grease the sides and base with butter. Secure the rim around the base of the pan. Add melted butter and sprinkle ⅓ cup of flour evenly over the pan.
Evenly alternate overlapping pieces of sweetpotato and pear until the bottom is covered.
In a medium bowl, shift together flour, baking powder, cinnamon, and salt. Set aside.
In a large bowl, mix together the remaining brown sugar, mashed sweetpotato, buttermilk, and eggs. Slowly, add the dry ingredients to the wet ingredients.
Carefully pour and spread the batter over the sweetpotatoes and pears. Bake in the oven for 40 minutes or until a wooden pick inserted in the center of the cake comes out clean.
Remove from the oven and allow the cake to cool for 10-15 minutes. Release the sides of the springform. Carefully flip the cake and remove the bottom pan. Slice and serve.