Preheat oven to 350°F. Grease and flour a 12-cup Bundt pan, tapping out the excess flour. You can also brush cake release goop on your pan as well to keep the cake from sticking. Set aside.
In a medium bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.
3 & 1/3 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon baking powder, ½ teaspoon salt
In a separate bowl or large measuring cup, combine the buttermilk, sour cream, and oil. Set aside.
½ cup buttermilk, ½ cup sour cream, ½ + 2 tbsp cup vegetable oil
In a large mixing bowl, cream the butter and sugar until light and fluffy (about 3–5 minutes). Add the eggs one at a time, beating well after each addition. Mix in vanilla.
2 sticks butter, 1 & 2/3 cups granulated sugar, 3 large eggs, 2 teaspoons vanilla extract
Add the dry ingredients to the butter mixture in three additions, alternating with the buttermilk mixture. Begin and end with the dry ingredients. Do not overmix.
Divide the batter evenly into three bowls:
Leave one bowl plain (white). Tint one bowl red. Tint one bowl blue. Drop spoonfuls of each color batter randomly into the Bundt pan. Once all the batter is in, gently swirl with a butter knife or skewer for a marbled effect.
1 teaspoon red food coloring, 1 teaspoon blue food coloring
Bake for 45-52 minutes, or until a skewer inserted into the center comes out clean. Let cool in the pan for 10–15 minutes before inverting onto a wire rack.
Once fully cooled, top with a dusting of powdered sugar or a drizzle with white chocolate ganache. Add festive sprinkles if desired and serve!