Preheat the oven to 350°F (175°C). Prepare a 9-inch round cake pan with butter and flour.
Whisk together the flour, baking powder, and salt in a large bowl.
In another bowl, beat the eggs, olive oil, milk, ricotta cheese, and grated Parmesan cheese until well combined.
Add the wet ingredients to the dry ingredients gradually, stirring until just combined. Do not overmix.
Pour the batter into the prepared cake pan and smooth the top.
In a separate bowl, toss the sliced zucchini, yellow squash, red bell pepper, and red onion with balsamic vinegar, salt, and pepper.
Arrange the marinated vegetables on top of the cake batter.
Bake for 40-45 minutes in a preheated oven, or until a toothpick inserted into the center comes out clean.
Remove from the oven and cool for 10 minutes in the pan. After that, place it on a wire rack to cool completely.
Garnish with fresh basil leaves before serving.