Preheat the oven to 350 degrees F. Line a 9x13-inch baking dish with parchment paper. Place the oats in a food processor and pulse to partially grind.
Place the brown sugar, butter, and vanilla extract in an electric stand mixer. Beat on high to cream the butter and sugar until light and fluffy, about 3-5 minutes.
Turn the mixer on low and add the eggs, baking powder, salt, and cinnamon. Then slowly add the flour and ground oats. Scrape the bowl, then beat again until smooth.
Scoop 60% of the dough into the baking dish. Spread the mixer evenly over the pan.
For the fudge layer, place the sweetened condensed milk, chocolate chips, and butter in a microwave safe bowl. Microwave for 2-3 minutes, until the mixture is warm enough to melt the chocolate. Stir until smooth and pour the fudge over the bottom oatmeal layer.
With the remaining oatmeal dough, drop small pieces over the entire surface for the fudge. Bake for 30-35 minutes, until golden on top. Cool completely before cutting. (Chill for 60 minutes for the cut pieces.)
NOTE: If you have a large food processor, you can make the entire oatmeal dough in it, without using a electric mixer.