Line an 9-inch square baking pan with parchment paper, leaving an overhang on the sides to lift the finished blondies out.
In a large mixing bowl, blend the melted butter, brown sugar, and vanilla together until combined. Add egg and mix well.
In another bowl, whisk the flour, baking powder, baking soda, and salt.
Slowly add the dry ingredients into the bowl of wet ingredients and stir to combine.
Spread evenly into prepared baking pan using a rubber spatula (dip in water if needed).
Bake for 20 minutes or until inserted toothpick comes out clean. Remove from the oven and allow to cool 15 minutes before spreading frosting.
Frosting
In a saucepan over medium heat melt the butter until it starts getting brown (butter is cooked long enough to turn the milk solids brown while cooking out any water present in the butter).
Stir in the dark brown sugar and continue cooking until the sugar dissolves and is bubbling. Remove from heat and stir in the heavy cream. This will steam and rise up. Add the vanilla, salt and powdered sugar. Mix thoroughly until blended and thick. Add more powdered sugar is you want a thicker frosting.
Spread butterscotch frosting on cooled blondies, spreading to the edges. Let cool completely or refrigerate for an hour, then lift blondies from pan by edges of parchment over hang and cut into 16 bars.