In a small saucepan, cook butter over medium heat until brown and fragrant. When brown specks (not black!) collect at the bottom of the pan, immediately transfer butter to a small bowl. Let cool to room temperature.
Preheat oven to 350ºF. Line the bottom of an 8 x 4-inch baking pan with parchment paper. Grease and flour the insides.
In a medium bowl, beat brown butter, brown sugar, eggs, vanilla and buttermilk until smooth.
Stir in flour mixture in two additions. Mix just until combined. A few lumps are okay.
Transfer batter to prepared baking pan. Bake until top is deep golden brown and toothpick inserted into center comes out clean, about 38-43 minutes.
Let bread cool in pan for 10 minutes, them turn out onto a cooling rack to cool completely. Top with two coats of butterscotch glaze.
Melt butter in a small saucepan over medium heat. Stir in brown sugar and salt.
Continue stirring over medium heat until mixture comes to a boil. Boil for two minutes.
Turn heat down and slowly stir in heavy cream. Keep stirring until mixture is smooth. Remove from heat and stir in vanilla extract.
Transfer mixture to a small bowl and let cool until thickened.