Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan—or do what I do and line it with parchment paper so it pops out like magic.
In a large bowl, whisk together the flour, 1 cup sugar, baking powder, and salt.
In a separate bowl, mix the milk, oil, eggs, and vanilla. Combine it with the dry stuff. Stir just until the batter comes together—don’t overthink it.
Pour half of the batter into the pan. In a small bowl, stir together 2/3 cup sugar and the cinnamon. Sprinkle half over the batter.
Add the rest of the batter, then sprinkle the remaining cinnamon-sugar over the top. Swirl it all gently with a knife—go wild, but not too wild.
Bake for about 45–50 minutes, or until a toothpick comes out clean. Let it cool (I know, it’s hard). Slice. Eat. Repeat.