Preheat the oven to 350 degrees. Line a 9X13-inch baking pan with aluminum foil, letting the foil hang over the each short edge by one or two inches. Lightly grease the foil with cooking spray.
Whisk together the flour, oats, baking soda, and salt and then set aside. Place the 3/4 cup butterscotch chips in a large bowl. Pour the hot melted butter over the butterscotch chips and whisk together until the chips are melted and smooth. Add the brown sugar and whisk until combined. Add in the egg and vanilla. Whisk until smooth. Stir in the flour mixture, just until incorporated.
Scrape the batter into the prepared pan and spread the batter evenly to the edges. Bake the bars until a toothpick inserted into the center comes out with just a few moist crumbs, this was about 16 minutes.
While the bars bake, place the 1/4 cup butterscotch chips, brown sugar, water and salt in a small microwave-safe bowl. Microwave the mixture until melted and smooth, this was about 2 minutes on 50% power. Whisk to combine well.
Slowly drizzle the glaze over the warm bars. Let the bars cool completely in the pan. I left the bars sitting on the counter overnight. Remove the bars from the pan using the foil overhang and cut into small squares.
These will keep well in an airtight container for several days.
The glaze recipe as written is perfect for drizzling LIGHTLY over the bars - this is my preferred way to make them. For a thicker glaze, as pictured in this post, you will need to double or triple the glaze recipe.As a result, the bars will be significantly sweeter this way. (My kids love these bars with the extra glaze, I'm partial to the very lightly drizzled version.)IF you are going to increase the amount of glaze to top these bars, you'll want to let the bars cool completely before pouring the glaze, so that you'll be able to spread it across the top of the bars with a spatula. If the bars are warm when you try to spread the glaze, it won't work well.