Preheat oven to 350F and prepare two 8" rounds with cake goop or preferred pan spray.
Whisk together flour, baking powder and salt and set aside
In the bowl of a stand mixer with paddle attachment, cream the butter for 20 seconds until smooth
Sprinkle in the sugar and whip on med-high until light in color and fluffy. About 8-10 minutes. Scrape the bowl halfway through mixing
Bring back to low mixing, add in eggs one at a time. Let mix in fully for one minute fore adding the next egg. Mixture should look smooth, not broken or lumpy. If it is, your eggs might have been too cold.
Add your vanilla to your milk
Add flour to mixture in four parts, alternating with the milk. Start with flour, mix until just incorporated, then add 1/3 of the milk. Repeat. After you add the last little bit of flour, STOP.
Take out about 1/3 of your batter and mix in your cinnamon. Finish the other batter by hand by mixing with a spatula until everything is combine. 3-4 strokes.
Spoon your batter into your two pans. Alternating between vanilla and cinnamon to get a nice swirl.
Bake for 25-30 minutes or until toothpick comes out clean. Do not over-bake or your cake will shrink and your sides will not be straight. Wrap your cakes in plastic wrap and chill them until firm before torting and frosting.