Preheat Oven and Prepare Pans: Heat your oven to 350°F (175°C). Grease two 9×5-inch loaf pans lightly with butter or nonstick spray.

Mix Dry Ingredients: In a large mixing bowl, whisk together 4 cups all-purpose flour, 2 teaspoons baking soda, and ½ teaspoon salt if using.

Cream Butter and Sugar: In another large bowl, use a handheld or stand mixer to cream 1 cup softened unsalted butter and 2 cups granulated sugar until light and fluffy.

Add Eggs and Vanilla: Beat 2 large eggs and 2 teaspoons vanilla extract into the creamed mixture until smooth.

Alternate Adding Dry Ingredients and Buttermilk: Gradually add the flour mixture and 2 cups buttermilk alternately into the creamed butter mixture, starting and ending with dry ingredients. Stir gently until just combined; avoid overmixing for tender bread.

Prepare Cinnamon Sugar Topping: In a small bowl, combine ¼ cup granulated sugar with 2 to 3 teaspoons ground cinnamon.

Layer Batter and Topping: Divide half of the batter evenly between the two loaf pans. Sprinkle about three-fourths of the cinnamon sugar evenly over the batter. Spread the remaining batter over this layer, then top with the remaining cinnamon sugar.

Create Swirls: Using a butter knife, gently swirl through the batter and cinnamon sugar layers to form a marbled pattern without mixing.

Bake the Bread: Place pans in the preheated oven and bake for 45 to 50 minutes or until a toothpick inserted in the center comes out clean or with just a few moist crumbs.

Cooling: Allow the bread to cool in the pans for 15 to 20 minutes before transferring to a wire rack to cool completely for best slicing texture.