Step 1: Preheat your oven to 350°F (175°C). Grease and lightly flour two standard loaf pans, ensuring they are well-coated to prevent sticking.
Step 2: In a large mixing bowl, cream together the softened butter, 2 cups of sugar, and eggs until the mixture is light and fluffy.
Step 3: In a separate medium bowl, whisk together the flour and baking soda.
Step 4: Gradually add the buttermilk (or milk/vinegar mixture) and the dry flour mixture to the creamed butter mixture, alternating between the two, beginning and ending with the flour. Mix until just combined, being careful not to overmix.
Step 5: In a small bowl, combine the 2/3 cup of sugar and 2 teaspoons of cinnamon for the swirl mixture. Stir well to ensure the cinnamon is evenly distributed.
Step 6: Pour about half of the batter (or a little less, roughly 1/4 of the total batter per pan) into the bottom of each prepared loaf pan. Sprinkle about three-quarters of the cinnamon-sugar mixture evenly over the batter in both pans. Gently add the remaining batter on top, dividing it between the two pans. Sprinkle the remaining cinnamon-sugar mixture over the top of the batter in each pan. Using a knife or a skewer, gently swirl through the layers a few times to create a marbled effect.
Step 7: Bake in the preheated oven for 45-50 minutes, or until a wooden toothpick inserted into the center of a loaf comes out clean. The top should be golden brown and the edges slightly pulled away from the pan.
Step 8: Remove the loaves from the oven and let them cool in their pans for 20 minutes. After 20 minutes, carefully invert the loaves onto a wire rack to cool completely before slicing and serving.