Prep Oven and Pan: Preheat oven to 350°F (175°C). Generously grease your Bundt pan (or 9x13 pan) with butter or baking spray.

Make Cinnamon Mix: In a small bowl, whisk together the 1/3 cup sugar and 1 tbsp cinnamon. Sprinkle about HALF of this mixture inside the greased pan, tilting the pan to coat the bottom and sides. Save the rest for the swirl.

Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, cream of tartar, and salt. Set aside.

Cream Butter & Sugar: In a large bowl or stand mixer fitted with the paddle attachment, beat the softened butter and 2 cups granulated sugar on medium-high speed for 3–4 minutes until pale and fluffy.

Add Wet Ingredients: Add eggs one at a time, beating well after each addition. Scrape down the sides of the bowl. Mix in the vanilla extract.

Combine Batter: Reduce mixer speed to low. Add one-third of the flour mixture, followed by half of the sour cream. Repeat (flour, remaining sour cream, remaining flour). Mix just until the flour streaks disappear. Do not overmix. The batter will be thick.

Layer: Spoon half of the batter into the prepared pan. Smooth the top. Sprinkle the remaining cinnamon-sugar mixture evenly over the batter. Top with the remaining batter and smooth gently to seal the edges.

Bake: Bake for 55–65 minutes (if using a Bundt pan) or 35–45 minutes (if using a 9x13 pan). The cake is done when a toothpick inserted into the thickest part comes out clean or with a few moist crumbs.

Cool: Let the cake cool in the pan for exactly 10–15 minutes. Invert onto a wire rack to cool completely. The sugar crust will be fragile while hot!