Preheat the oven. Preheat to 350°F. Grease a 9x5 cast iron loaf pan or standard 9×5 loaf pan.
Mix the dry ingredients. In a medium bowl, whisk together the freshly milled soft white wheat flour, baking powder, baking soda, and salt. Set aside.
Mix the wet ingredients. In a large bowl, whisk together all wet ingredients, butter, sugar, eggs, milk and vanilla.
Combine. Add the dry ingredients to the wet ingredients and gently stir just until combined. Do not overmix.
Make the cinnamon swirl. In a small bowl, mix together the sugar, cinnamon, and melted butter.
Layer the batter. Pour about 3/4 of the batter into the prepared loaf pan and spread evenly. Sprinkle most of the cinnamon sugar mixture over the batter, reserving about a tbsp or two for the topping. Add the remaining batter, then sprinkle the rest of the cinnamon mixture evenly over the top.
Swirl: Use a knife or skewer to gently swirl the cinnamon mixture through the batter. I start at one end and do a ribbon pattern to the other end.
Bake: Bake for 50–55 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs. After about 45 minutes of baking, I like to tent it with foil to keep the top from browning too much.
Cool: Let the bread cool in the pan for 10–15 minutes, then transfer to a wire rack to cool completely before slicing.