Preheat the oven to 375 F.
Butter or spray your pan.
In a large bowl, combine oats, walnuts (save 1/3 cup for top), cinnamon, baking powder, and salt.
In a separate bowl, mix together milk, heavy cream, vanilla, eggs, and maple syrup.
Pour the wet ingredients over the top of the oat mixture and stir to make sure everything is incorporated and there are no dry spots.
Bake for 40-45 minutes, until golden brown and no liquid is left on the surface.
To make the frosting, start by mixing the cream cheese and butter until smooth.
Add in the powdered sugar and heavy cream or coffee creamer.
Let the oatmeal bars cool for at least 10-15 minutes before cutting into squares and drizzling the top of the bars with frosting.
Sprinkle on extra walnuts.