Mash the bananas and then add enough milk to make one total cup (if necessary). Stir in the yogurt and set aside. Whisk together the flour, sugar, baking powder, salt, and cinnamon in a large bowl. Cut the butter into several pieces. Work it into the dry ingredients using a fork, pastry cutter, or your finger tips until there are no pieces of butter larger than a pea. Pour the banana-yogurt mixture into the bowl with the flour and stir just enough to incorporate all of the flour. Fold in the walnuts, if using. This will make a fairly wet dough. Line a dinner plate with a piece of wax paper and turn the dough out on top. Pat it into a disk about 1-inch thick and cover with another piece of wax paper. Freeze the scone dough for 30 minutes. Preheat the oven to 400?F. Peel off the top layer of wax paper and invert the scones onto a parchment-lined baking sheet. Peel off the second layer of wax paper. Slice the scones into eight wedges and pull them apart a little to give them some room to expand. Bake for 25-30 minutes, until the scones are firm to the touch and turning golden-brown on the edges. Cool completely and cut apart any scones that baked together with a sharp knife. To make the glaze, melt the butter and the milk in the microwave for 30 seconds. Add the brown sugar and vanilla, and stir until the sugar has melted (heat for an additional 30 seconds in the microwave if necessary). Whisk in the confectioner's sugar, starting with 1/4 cup. Add more confectioner's sugar if desired to make a thicker glaze. Just before serving, drizzle the glaze over the scones. The glaze will harden after setting for a minute or two, and can be served right away or packed for a later snack. The glaze can make the scones a bit sticky if kept for longer than a few hours, so store any scones to be eaten later un-glazed. Extra un-glazed scones can also be frozen and re-heated in a microwave or toaster oven.