Line an 8 x 8 (or larger) baking dish with parchment paper and set aside.
Make the base: in a large mixing bowl combine 3 cups rolled oats, 1 cup peanut butter, and 3/4–1 cup maple syrup; mix until fully combined and press evenly into the prepared pan. Refrigerate while preparing the fudge layer.
Prepare the fudge center: in a microwave-safe bowl or small saucepan, gently heat 2 cups peanut butter with 1 cup maple syrup until pourable; stir in 1½ cups oat flour until a thick fudge batter forms.
Pour the fudge batter over the chilled base, spread evenly, and return to the refrigerator to firm up.
When the bars are firm, spread 1 serving of chocolate frosting over the top and refrigerate about 30 minutes more, or until the frosting is set.
Lift the bars from the pan using the parchment, slice into 24 bars, and serve.