Preheat the oven to 350°F (175°C). Grease and flour a cake pan.

In a mixing bowl, cream together the softened butter and sugar until light and fluffy.

Add eggs one at a time, beating well after each addition. Stir in the vanilla extract.

In a separate bowl, combine flour, baking powder, and salt. Gradually add to the creamed mixture, mixing until just combined.

Fold in the mixed berries, banana slices, and diced apple into the batter.

Pour the batter into the prepared cake pan and smooth the top with a spatula.

Bake in the preheated oven for 30 minutes or until a toothpick inserted into the center comes out clean.

Let the cake cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Serve slices of the cake topped with whipped cream.