Preheat oven to 350ºF. Line inside of an 8-inch square metal baking pan with foil and grease foil. For thinner bars, you can try a 9-inch square pan.
Cream the butter, salt and both sugars. Stir in the syrup, vanilla and the oats. Press mixture into the pan and bake for about 18 minutes (15 if using a 9-inch pan) or until edges are lightly browned.
Let cool. It's best to let it cool completely, but I'll admit to putting the topping on while these were still warm.
Topping: put the chocolate chips and peanut butter in a small microwave-safe bowl. I like to use my 2 cup Pyrex measuring cup. Heat for 30 seconds. Stir until fully melted. Depending on the power of your microwave, you might need to heat for another 10-20 seconds to fully melt the chips.
Pour the chocolate over the bars and forget about them until the chocolate sets. To speed things up, you can put them in the refrigerator. I thought these tasted better chilled. Lift from pan and cut into 16 squares.