Some people love their banana bread super-duper moist. If that's the case, I recommend putting 1 3/4 cup of mashed banana. Also, a few tips with the icing: on Tasty Kichen there was much debate about whether or not the butter should be room temperature and I found that it should be a little bit in between. It shouldn't be super soft, meaning that if you cut the butter with a plastic knife it should give a little but not turn to mush. If it's too melted then the sugar won't properly incorporate and you'll have grainy icing on your hands. Not fun.
1 1/2 cups all-purpose flour (spooned and leveled)
3/4 cup sugar
1 teaspoon baking powder
1/2 teaspoon baking soda
1/4 teaspoon salt
1/2 cup (1 stick) unsalted butter, melted
1 1/2 cups mashed bananas (about 4 ripe bananas), plus 1 whole banana, for garnish (optional)
2 large eggs
1/2 teaspoon pure vanilla extract
1/4 cup of chopped walnuts or pecans
1 cup Milk
5 Tablespoons Flour
1 teaspoon Vanilla
1 cup Butter, barely room temperature
1 cup Granulated Sugar (not Powdered Sugar!)
Since the icing has 2 steps, I'd start it first. So in a small saucepan, whisk flour into milk and heat, stirring constantly, until it thickens. Remove from heat and let it cool to room temperature. (If you're in a hurry, place the saucepan over ice in the sink for about 10 minutes or so until the mixture cools.)
Then turn your attention to the cupcakes and preheat the oven to 350 degrees. Line a standard 12-cup muffin pan with paper liners. In a medium bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
Make a well in center of flour mixture. In well, mix together butter, mashed bananas, eggs, and vanilla. Stir to incorporate flour mixture (do not overmix). Add chopped walnuts and fold in. Dividing evenly, spoon batter into muffin cups (I filled them up about 3/4 of the way full) and place in preheated oven. You're going to want to bake them for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.
While the cupcakes are in the oven, go back to the icing and cream the butter and sugar together until light and fluffy. Add the vanilla to the cooled milk/flour mixture. Then add the cooled milk/flour/vanilla mixture and beat until it all combines and resembles whipped cream.
NOTE: The milk/flour/vanilla mixture MUST be cooled before you add it to the butter and sugar mixture. Also, if you'd like to see the step-by-step post that Ree did about the icing, then click here.
Remove cupcakes from pan; cool completely on a wire rack. Spread tops with frosting. Just before serving, peel and slice banana into rounds, and place one on each cupcake.