In a small saucepan, bring brown sugar and butter to a boil over medium-high heat. Boil for 1 minute. Remove from heat, and let cool for 15 minutes. Stir in warm milk.
In the bowl of a stand mixer fitted with the whisk attachment, beat brown sugar mixture, confectioners’ sugar, chocolate-hazelnut spread, and salt at medium speed until lightened in color, about 2 minutes. Use immediately.
Notes
*We used Nutella.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/banana-cake-brown-sugar-hazelnut-buttercream/