Preheat oven to 400℉ with rack in the upper middle position.
If making the buttermilk substitute, combine the milk and lemon juice and set aside.
Measure out 2 cups of flour in a large bowl. REMOVE 1 Tablespoon of flour from these two cups. Add the cornstarch, sugar, baking soda, cream of tartar, salt, and cinnamon. Whisk to combine.
Using a fork, cut the butter into the flour in a corner of the bowl until the flour is broken down and it looks like wet sand. The butter should not be in large pieces or strips anymore.
1 Tablespoon unsalted butter
Add the buttermilk (or milk mixture) and raisins to the flour and use a fork to combine.
3/4 cup buttermilk, 1/2 cup raisins
Pour out the dough and lightly knead just until everything is combined. It should not be completely smooth.
Pat the dough down into a disk approximately 1 inch tall and 5 inches wide. Transfer to a parchment or silicone mat lined baking sheet.
Using a serrated bread knife, cut an X in the top of the dough approximately 1/2 to 1 inch deep.
Bake the bread until golden brown and a toothpick inserted in the middle comes out with just a few crumbs, approximately 25 minutes.
Let the bread cool on a cooling rack for 1 hour.
Notes
You can easily double all the ingredients for a larger, "regular" loaf. Make the disc in step 7 be 2 inches high and 6 inches wide, and cook for closer to 40 minutes.If you do not have buttermilk, combine 3/4 cup whole milk with 2 teaspoons lemon juice and set aside for 5 minutes.