Preheat and Prepare the Pan:
Preheat the oven to 350°F (175°C). Grease and flour the bundt pan or cake pan to ensure easy release.
Mix Dry Ingredients:
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
Cream Butter and Sugar:
In a large bowl, beat the butter and sugar together until light and fluffy, about 3–4 minutes.
Add Wet Ingredients:
Beat in the eggs one at a time, followed by the vanilla extract, orange zest, and lemon zest.
Combine Wet and Dry Ingredients:
Alternate adding the dry ingredients and buttermilk to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined.
Fold in Berries:
Gently fold the mixed berries into the batter, being careful not to overmix.
Bake:
Pour the batter into the prepared pan and smooth the top. Bake for 45–50 minutes, or until a toothpick inserted into the center comes out clean.
Cool the Cake:
Allow the cake to cool in the pan for 10–15 minutes before turning it out onto a cooling rack. Let it cool completely.
Prepare the Glaze:
In a small bowl, whisk together the powdered sugar, orange juice, and lemon juice until smooth.
Glaze the Cake:
Drizzle the glaze over the cooled cake, letting it drip down the sides.
Garnish:
Decorate with fresh berries, citrus slices, and mint leaves, if desired.