Whisk the flour, baking soda, cinnamon, and salt together. Set aside.
Using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter and both sugars together on high speed until smooth and creamy, about 2 minutes.
Add the eggs, yogurt, and vanilla, then beat on medium-high speed until combined.
Beat in the mashed banana.
With the mixer on low speed, add the dry ingredients until just incorporated.
With the mixer still running on low, slowly pour in the buttermilk until combined. Add the chopped walnuts. Do not overmix. You may need to whisk it all by hand to make sure there are no flour pockets at the bottom of the bowl.
Pour/spoon the batter into the liners – fill only 2/3 full to avoid spilling over the sides. Bake for 18-20 minutes, or until a toothpick inserted in the centre comes out clean. Allow the cupcakes to cool completely before frosting.