Pre heat oven to 350 degrees F.
In a large bowl, mix together the butter and sugar. Cream butter and sugar together by mixing together for 1-2 minutes (medium speed on stand mixer).
Add wet ingredients (milk, eggs, and vanilla) to the butter and sugar.
Add the dry ingredients (salt, flour, and baking powder) and mix together until the batter is smooth.
Prepare the 9×13 cake pan by rubbing the sides and bottom with butter.
Pour the cake batter into the prepared baking pan. Use a spatula to get any remaining batter out of the bottom of the bowl.
In a separate bowl, combine the sugar and cinnamon. Sprinkle the cinnamon sugar across the top of the batter in an even layer.
Bake for 30-35 minutes.
For best results, bake until the center of the cake is set and the top of the cake turns golden brown.