Preparation Steps

• Preheat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line with parchment if desired.
• In a medium bowl, whisk together the flour, cream of tartar, baking soda, and salt until well combined.
• In a large bowl, cream the softened butter and 1.5 cups granulated sugar until light and fluffy, about 2 to 3 minutes.
• Beat in the eggs one at a time, then add the vanilla and mix until incorporated.
• Add the dry ingredients to the butter mixture in three additions, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined; do not overmix.
• In a small bowl, combine the remaining 2 tablespoons sugar and 2 tablespoons cinnamon to make the cinnamon-sugar mixture.
• Pour half of the batter into the prepared pan. Sprinkle half of the cinnamon-sugar mixture evenly over the batter. Top with the remaining batter and sprinkle the rest of the cinnamon-sugar on top. Use a knife to gently swirl the layers for a marbled effect.
• Bake in the preheated oven for 45 to 55 minutes, or until a toothpick inserted into the center comes out clean. If the top browns too quickly, tent with foil.
• Allow the bread to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.