To make the butterscotch: In a medium saucepan over medium heat, melt the butter. Whisk in the brown sugar, then whisk in cream then let mixture bubble and thicken for 5 minutes without stirring. Remove from heat and whisk in vanilla and salt. Allow to cool.

Preheat oven to 350F degrees. Line 12-count cupcake pan with liners or spray with nonstick spray. Set aside.

To make the cupcakes: Whisk the flour, baking powder, baking soda, and salt together in a medium bowl. Set aside. In a large microwave-safe bowl, melt butter in the microwave. Whisk in the brown sugar vigorously until no lumps remain. Whisk in egg, yogurt, milk, and vanilla extract until combined. Slowly mix in dry ingredients until no lumps remain. Batter will be thick. Divide batter between 12 cupcake liners. Bake for 20 minutes or until a toothpick inserted in the center comes out clean. Allow to cool completely before filling.

To make the frosting: In a medium bowl, beat softened butter on medium speed with an electric or stand mixer fitted with a paddle attachment. Beat for 3 minutes until smooth and creamy. Add powdered sugar, cream, and vanilla extract with the mixer running. Increase to high speed and beat for 3 minutes. Add more powdered sugar if frosting is too thin or more cream if mixture is too thick. Add 1/4 teaspoon of salt (or more) if frosting is too sweet.

Fill & frost the cupcakes: Fill a plastic squeeze bottle or piping bag (with a filling tip) with butterscotch sauce and push tip of bottle into top of each cupcake, applying enough pressure to fill each cupcake with sauce. Top with vanilla frosting and drizzle with remaining butterscotch sauce. I used a Wilton 1M tip for my frosting.

Cupcakes remain fresh covered at room temperature for 2 days or in the refrigerator for 4 days. Store any leftover butterscotch in the refrigerator for up to 1 week.