Step 1: Preheat your oven to 350°F (175°C) and generously grease a 10-cup bundt pan with non-stick spray or butter, then lightly flour it.
Step 2: In a small saucepan over medium heat, melt the 1/4 cup unsalted butter and brown sugar together, stirring until combined and bubbly. Remove from heat.
Step 3: Stir in the ground cinnamon until well blended.
Step 4: Pour the butter mixture evenly into the bottom of the prepared bundt pan. Arrange the sliced apples and mixed berries over the butter mixture in a decorative pattern.
Step 5: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.
Step 6: In a large mixing bowl, using an electric mixer, cream the 1/2 cup softened unsalted butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Step 7: Beat in the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
Step 8: Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Begin and end with the dry ingredients, mixing on low speed until just combined. Do not overmix.
Step 9: Carefully pour the cake batter over the arranged fruits in the bundt pan, gently spreading with a spatula to ensure an even layer.
Step 10: Place the bundt pan in the preheated oven and bake for 45-55 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Step 11: Once baked, remove the cake from the oven and let it cool in the pan for about 10-15 minutes. This cooling period is crucial for the cake to set and for the fruit topping to caramelize slightly and release easily.
Step 12: Carefully invert the cake onto a serving plate. If it doesn’t release easily, gently tap the pan on the counter or use a thin knife to loosen the edges before inverting. Allow the cake to cool slightly before slicing and serving warm or at room temperature. Enjoy!