Preheat and prepare pan. Position a rack in the center of your oven and preheat the oven to 350°F (177°C). Spray a 9”x5” loaf pan with cooking spray, or line with parchment paper and then spray with cooking spray.
Make the cinnamon swirl. In a small mixing bowl, stir together the sugar and cinnamon. Set aside.
Combine dry ingredients. In a mixing bowl, whisk together all-purpose flour, baking powder, and salt. Set aside.
Combine wet ingredients. In a second mixing bowl, whisk together brown sugar and eggs until smooth. Whisk in the buttermilk ,oil, and vanilla until combined.
Combine wet and dry ingredients. Add the dry ingredients to the wet ingredients and mix together with a spatula or wooden spoon just until just combined. Fold in butterscotch chips(I toss them in a tablespoon of flour first so they don’t sink in the batter when baking). Do not overmix. Stop mixing once the last of the flour has been incorporated (batter will not be completely smooth).
Assemble the bread. Spoon ½ of the batter into prepared loaf pan. Sprinkle on almost of all of the cinnamon sugar mixture, except 2 Tablespoons. Spoon the rest of the batter on top. Sprinkle on the remaining cinnamon sugar
Bake. Bake in a preheated 350°F (177°C) oven for 55-65 minutes until a toothpick inserted into the middle of bread comes out clean or with a few moist crumbs clinging to it (no wet batter) and top springs back to the touch when lightly touched. Remove from the oven and cool completely in the pan on a wire cooling rack.
Make glaze. The glaze made too early will harden so do not make until ready to drizzle over the bread. In a saucepan over low heat stir together the butter and butterscotch chips, until melted. Remove from the heat and Stir in vanilla, milk and powdered sugar until sugar is dissolved and mixture is smooth. Drizzle over cooled bread immediately. If your glaze hardens, simply remelt over low heat until smooth and melted again.