Beat butter, sugars, eggs, vanilla, and peanut butter in an electric mixer until smooth. Add flour, baking soda, and salt. Add oats and stir until just moistened. Spread onto a 11 x 17-inch baking sheet that has been greased or covered in parchment paper.
Smooth until entire pan is covered evenly. Bake at 375 F 15 to 20 minutes or until lightly browned.
Meanwhile, mix all icing ingredients together in mixer until smooth.
Let baked mixture cool, then ice and cut into bars.
Per serving: 433 calories, 24 grams fat (11 grams saturated fat), 48 grams total carbohydrates, 2.3 grams fiber, 8 grams protein.