Review all recipe notes and instructions before beginning.
Preheat the oven to 350 degrees.
In a microwave safe bowl, combine the sweetened condensed milk and the chocolate. Microwave for 25-30 seconds and stir. Microwave for an additional 15 seconds. Stir until the chocolate melts.
In a stand mixer fitted with the paddle attachment (or large bowl with hand mixer), add the chocolate condensed milk mixture, brown sugar, and vanilla. Mix on medium until well combined.
Add the eggs and mix on medium low until combined.
Add the flour, cocoa powder, and salt. Fold in with a spatula or mix on the lowest speed until just combined.
In a microwave safe bowl, melt the butter. Add the flour, oats, sugar, M&Ms, and peanut butter chips. Fold together until just combined.
Line an 8 x 8 metal pan lined with parchment paper and grease lightly with nonstick spray. Pour the brownie batter in. Top with oatmeal crumble.
Bake for 35-40 minutes, or until just set. The middle should still be slightly sticky on a toothpick.
Let cool on a wire rack for 2 hours or until completely set. Remove and slice.