Follow these steps to create your Blueberry Nectarine Upside-Down Cake effortlessly:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan.
2. Prepare the Fruit: Arrange the sliced nectarines and blueberries evenly at the bottom of the prepared cake pan.
3. Make the Topping: In a small saucepan, melt 1/4 cup of the butter with 1/2 cup of granulated sugar over low heat. Stir until combined, then pour this mixture over the arranged fruit in the pan.
4. Cream Butter and Sugar: In a large mixing bowl, beat the remaining 1/4 cup of softened butter with 1/2 cup of granulated sugar until light and fluffy.
5. Add Eggs and Vanilla: Beat in the eggs one at a time, then add the vanilla extract, mixing until fully incorporated.
6. Combine Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, and salt.
7. Mix Together: Gradually add the dry ingredients to the wet mixture, alternating with the milk. Mix until just combined.
8. Pour Batter: Carefully pour the batter over the fruit layer in the cake pan, spreading it gently to cover.
9. Bake: Place the cake in the preheated oven and bake for 35-40 minutes. A toothpick inserted into the center should come out clean.
10. Cool: Remove the cake from the oven and let it cool in the pan for about 15 minutes.
11. Flip the Cake: Carefully invert the cake onto a serving platter, allowing the fruit to be on top.

Following these steps will yield a beautiful and delicious Blueberry Nectarine Upside-Down Cake that is sure to impress.