In the bowl of a stand mixer, cream together the butter and sugars. Add the egg, peanut butter, baking soda, salt and vanilla. Mix well. Stir in the flour and oats. Mix just until combined. Spread in an 8x8 pan that has been sprayed with cooking spray.
In a small bowl, combine the ingredients for the streusel. Mix until the mixture is crumbly. Sprinkle over the top of the batter. Bake at 350 degrees for 25-28 minutes.
Once the bars have cooled to room temperature, mix the powdered sugar and peanut butter in a small bowl. Add the milk, a teaspoon at a time, until the mixture becomes a drizzling consistency.
Pour the peanut butter drizzle over the oatmeal bars. Allow the drizzle to set for 10 minutes, then serve.
Store any leftovers in an airtight container.
Notes
If you want the bars to come out cleanly for easy and neat cutting, line the baking pan with parchment paper before pressing the batter into the pan.