Preheat oven to 375 degrees F. Line a 9-inch x 13-inch baking dish with foil, and then coat the foil with non-stick cooking spray. Set aside.
In a medium saucepan, melt butter, brown sugar, and corn syrup over medium heat, until sugar is dissolved, about 3 to 4 minutes.
Remove pan from heat. Add half the peanut butter and vanilla. Stir until peanut butter is melted and smooth.
Place oats in a large mixing bowl. pour sugar mixture over oats and stir until oats are well coated.
Pour oat mixture into prepared baking dish. Use a spatula to press the oat mixture evenly into the dish.
Bake for 15 to 18 minutes, until edges are bubbly.
While oats bake, melt milk chocolate and semi-sweet chocolate in the same medium saucepan over low heat until smooth. Add remaining peanut butter and stir until smooth and incorporated.
Once you remove the oats from the oven, sprinkle half the pecans on top of the oats. Pour chocolate mixture over the top of the oats and pecans, and spread evenly. Sprinkle the top of the chocolate with the remaining pecans.
Transfer to a wire rack to cool for 20 to 30 minutes. Once the dish has cooled to the touch, transfer to the fridge until chocolate is completely set.
Remove from fridge, and lift bars from the pan using the foil. Carefully peel the foil off the block. Cut the block into bars. Cover and refrigerate until ready to serve.
I cut the oat block in half lengthwise and then cut the whole block into 16 bars, 8 from each half created by the first cut. You could cut them smaller and get many more bars.