Step 1: Preheat oven to 350°F (175°C) and grease a 10-12 cup Bundt pan thoroughly with baking spray or butter and flour.
Step 2: Whisk together cake flour, baking powder, and salt in a medium bowl.
Step 3: Cream 1 cup of room-temperature butter and granulated sugar in a stand mixer for 5 minutes until very light and fluffy.
Step 4: Add eggs and egg whites one at a time, followed by 2 tsp mint extract and 1 tsp vanilla extract, mixing well.
Step 5: Combine buttermilk and sour cream in a small bowl.
Step 6: Alternately add dry ingredients and buttermilk mixture to the butter, beginning and ending with flour.
Step 7: Remove 1 cup of batter and tint it green; layer white and green batter into the pan and swirl with a knife.
Step 8: Bake for 45-55 minutes, then cool in the pan for 15 minutes before inverting onto a wire rack to cool completely.
Step 9: For the frosting, beat 2 cups cold butter until smooth, then gradually add powdered sugar, cream, vanilla, and salt until fluffy.
Step 10: Frost the cooled cake and garnish with green sprinkles if desired.