Preheat the oven to 375℉. Butter a 9-inch round cake pan and line it with a circle of parchment paper.
In a large bowl, blend the butter and sugar with a mixer, or by hand, until it is light and fluffy. Add the eggs one at a time, then add the vanilla.
In a medium bowl, sift together the flour, baking powder, salt and cinnamon.
Add the flour mixture and the milk to the butter mixture in three additions, starting with ½ cup of flour and ⅓ of the milk, and ending with the remaining flour. Stir well after each addition.
Spoon the batter into the cake pan, and smooth the top with a spatula. Bake for about 30 minutes or until a toothpick inserted in the cake comes out clean. Cool the cake in the pan on a wire rack for at least 20 minutes before turning it out to cool completely.