1/2 pound softened unsalted butter
1 3/4 cups light brown sugar
5 large eggs at room temperature
5 ounces bittersweet chocolate, melted
1 tsp. vanilla extract
1 cup all-purpose flour
1/2 cup finely chopped macadamia nuts
1 medium overripe banana, mashed
1 recipe White Chocolate Frosting

Preheat oven to 350°. Line a 12-well muffin pan with paper liners; set aside. In a large bowl and using an electric mixer at medium-high speed, beat butter and sugar together until light and fluffy, about 5 minutes. Add eggs, one at a time, beating well after each addition. Add chocolate and vanilla extract, and mix well. In a small bowl, combine flour and nuts. Alternately add flour mixture and mashed banana to chocolate mixture, beginning and ending with flour. Divide batter evenly among muffin cups; bake for 30 to 35 minutes, or until a wooden pick inserted in centers comes out clean. Cool cupcakes in pan 5 minutes, then transfer to a wire rack to cool completely. Top with White Chocolate Frosting.
White Chocolate Frosting

1 cup softened unsalted butter
2 cups confectioners’ sugar
1/8 tsp. salt
7 ounces good-quality white chocolate, melted and cooled
4 T. heavy whipping cream
1 tsp. clear vanilla extract (if you can find it – keeps the frosting very white)

In a large bowl, and using an electric mixer at medium-high speed, beat butter, confectioners’ sugar, and salt together for 5 minutes, or until light and fluffy. Add white chocolate, cream, and vanilla extract. Beat at high speed for 5 to 6 minutes, periodically scraping down sides of bowl, until smooth and thick.