Preheat oven to 350 degrees F. Line a muffin tin with cupcake liners.
Combine flour, salt, baking soda, cinnamon and nutmeg in a bowl and set aside.
In a larger bowl, whisk egg and add brown sugar, mixing until smooth. Stir in vanilla extract. Add sour cream and melted butter and mix. Stir in dry ingredients, then add mashed bananas and milk, mixing until batter comes together. The batter will be slightly thick. Using an ice cream scoop or 1/4 cup measure, scoop batter in liners until 2/3 of the way full. Bake for 15-18 minutes. Let cool completely.
In the bowl of an electric mixer, beat butter until smooth and creamy. With the mixer on low speed, gradually add in sugar until it’s combined and all added. Add in vanilla and 2 tablespoons of bourbon, then increase the speed to medium-high and beat until fluffy, about 3-4 minutes. If the frosting still seems too thick (or you just, uh, want more booze), add in remaining bourbon and more if necessary.
bourbon chocolate ganache
Add chocolate to a large bowl. Heat heavy cream until just hot – when bubbles are appearing on the edges, then remove from heat and pour over chocolate. Stir constantly until the chocolate is melted and smooth. Once creamy, add in the bourbon, mixing again until smooth. Use however you’d like – dipping, drizzling, etc. I chose to dip about 1/3 of each cupcake top in the ganache before it firmed up, then topped it with some crushed toffee pieces.