With mixer on medium-low speed, add the unsalted butter a few pieces at a time, mixing well after each addition, then
add the brown butter in a few spoonfuls at a time. Once all butter has been added switch to the paddle attachment,
add vanilla and maple reduction and continue mixing on low speed until all air bubbles are eliminated, about 2
minutes. Scrape down sides of bowl with a flexible spatula, and continue mixing until the frosting is completely
smooth.