• Preheat oven to 350 degrees F.
• Add the butter, sugar and brown sugar to a mixing bowl and cream together until very smooth.
  ½ cup salted butter, 1/2 cup granulated sugar, ½ cup light brown sugar
•  Add egg and vanilla and mix well.
  1 large egg, 1 teaspoon vanilla extract
• Add flour and baking powder and stir until combined. 
  1 1/4 cups all-purpose flour, ½ teaspoon baking powder
• Fold in butterscotch chips (keep a small handful to sprinkle on top).
  ½ cup butterscotch chips
• Spread into a greased square (8 or 9 inch) baking pan and bake for about 25 minutes, or until a toothpick inserted comes out clean. 
• Allow to cool before cutting into bars.

For the Butterscotch Sauce:

• Melt the butter in a saucepan over medium heat. 
  ½ cup salted butter
• Add the sugar and cream and stir well with a wooden spoon until blended. 
  1 cup dark brown sugar, 1 cup heavy whipping cream
• Bring mixture to a boil, reduce heat and boil for about 5 minutes, stirring often.
• Remove from heat and stir in vanilla. The sauce will thicken as it cools. 
  3 teaspoons vanilla extract
• Serve warm over ice cream.
• To store, refrigerate sauce in an air-tight container.