Cinnamon Gooey Butter Cake is a soft, rich, and tender cake, covered with buttery, snickerdoodle-like cinnamon-sugar goo. It's addictive!
Combine 8 tablespoons butter with ¾ cup sugar and beat in a stand mixer or with a hand mixer until light and fluffy. Add milk and egg and beat until combined, then an extra 10 seconds more. Add 1 ½ cups flour, cream of tartar, baking soda, and salt, and beat on low until just combined.
Bake at 350F for 25-30 minutes (27 is perfect in my oven). The center will still be liquidy and jiggle; this will make goo. Cool completely, then cut into several little squares.
adapted from the Smitten Kitchen Cookbook