Make the oatmeal crust. Use a fork to whisk together the melted butter, brown sugar, white sugar, and orange zest in a large bowl. Set aside. In a small bowl, whisk an egg with pure vanilla extract until frothy. Mix the egg into the butter and sugar mixture. Mix in the flour, baking powder, salt, and cinnamon to the wet ingredients. Add in the oats and mix together.
1/2 cup unsalted butter, 1/4 cup brown sugar, 1/4 cup white sugar, 1 teaspoon orange zest, 1 egg, 1 tablespoon pure vanilla extract, 1 cup all-purpose flour, 1 ⅔ cups rolled oats, 1 teaspoon baking powder, 1/4 teaspoon salt, 2 teaspoons cinnamon powder