Preheat oven to 400 degrees and line a baking sheet with parchment paper.
Mix flour, cinnamon, and baking powder together. Add in cold butter, and mix with a pastry cutter until the mixture resembles coarse crumbs.
Add in the mashed banana, vanilla extract, maple syrup, milk, and sour cream (or Greek yogurt), and mix until combined.
Turn the dough out onto a floured surface, and pat the dough into a circle, about 1" thick. Cut into 8 equal triangles.
Press walnuts onto the tops of each scones.
Bake for 16-17 minutes, or until lightly golden brown.
While the scones are cooling, mix butter and cream cheese together until combined. Add in maple syrup and mix until combined. Put cream cheese icing into a pastry bag and drizzle over each scone.
Enjoy!