Preheat oven to 350°F. Line an 8-by-8-inch pan with aluminum foil or parchment paper and spray with cooking spray; set aside.

Melt ½ cup butter in the microwave. Combine remaining brown sugar and melted butter in a large bowl and whisk to combine. Add the egg, vanilla, and whisk to combine. Add the flour, salt, cinnamon and nutmeg and stir until just combined, taking care not to over-mix. Gently stir in the banana. Fold in the pecans. Pour batter into prepared pan, smoothing it lightly with a spatula or offset knife.

Bake for 30 minutes, or until the bars are set. The edges will be slightly pulling away from sides of pan. Let cool while you make the frosting.

Melt butter in a medium saucepan. Stir in brown sugar and milk and stir until it comes just to a boil. Remove from heat and allow to cool for 5 minutes. Whisk in powdered sugar and cinnamon.

Spread on top of blondies and let frosting set. Allow bars to cool for at least 2 hours before slicing and serving into 16 squares. I like to refrigerate them before cutting to make them easier to cut. Bars can be stored in an airtight container at room temperature or in the refrigerator for up to 1 week, or in the freezer for up to 3 months.