Preheat oven to 350 degrees.
In a medium bowl, whisk together the milk, egg, vanilla, maple syrup, melted (and slightly cooled) butter, vanilla with a whisk or handheld mixer until fully combined.
Add the oats, salt, baking powder, and cinnamon then mix well to combine.
Let the mixture rest for 5-10 minutes while you grease the 9x13 baking dish. The rest time is optional, but it allows the oats to absorb the liquid and helps give the oatmeal a more creamy and custardy consistency.
After the mixture has rested, add it to the prepared greased 9x13 pan.
Bake for 35-38 minutes, until the edges are slightly golden and the middle is set.
Let cool for 10-15 minutes before slicing and serving. This helps it set up and makes for easier slicing.
Enjoy!