Heat oven to 350°F. Line a 12-cup muffin pan with paper liners.
Toast pecans on a baking sheet for 5–7 minutes until fragrant. Cool, then chop.
Whisk flour, baking powder, baking soda, salt, and cinnamon in a bowl.
Whisk melted butter with brown sugar and granulated sugar until glossy and combined.
Stir in mashed bananas, beaten eggs, and vanilla until smooth.
Add dry ingredients in two additions, alternating with buttermilk. Mix just until no flour streaks remain.
Fold in 3/4 cup chopped pecans. Divide batter into liners, about 3/4 full.
Bake 16–19 minutes, until a toothpick comes out clean. Cool 5 minutes, then transfer cupcakes to a rack to cool completely.
Beat cream cheese and softened butter until creamy and smooth.
Gradually mix in powdered sugar, vanilla, and salt. Beat until light and fluffy, 1–2 minutes.
Frost cooled cupcakes and sprinkle with the remaining pecans.