Upside-down cake with nectarines and salted caramel

Ingredients:

• 1 and 3/4 cups sugar
• 1 teaspoon fine salt
• 4 nectarines, sliced ​​into 1cm-thick wedges (about 0.6 kg)
• 1 and 3/4 cups premium flour
• 1 teaspoon baking powder
• 0.5 tsp of soda
• 110 g unsalted butter, room temperature
• 2 large eggs
• 1 tsp vanilla extract
• 1 cup of sour milk or kefir
• Coarse sea salt, for sprinkling

Preparation:

1. Preheat oven to 375°F (190°C) and spray a 9x9 inch (22x22cm) loaf pan with cooking spray.
2. In a medium saucepan, combine 3/4 cup sugar, 1/2 teaspoon fine salt, and 1 1/2 tablespoons water and stir until the mixture resembles wet sand. Heat over medium heat, gently swirling the pan occasionally but not stirring with a spoon, until the caramel turns light amber, about 5 minutes. Remove from heat and immediately pour into the prepared pie pan (the caramel sets quickly). Arrange the nectarines on top of the caramel in 4 rows; set the pan aside.
3. In a bowl, combine the flour, baking powder, baking soda, and 1/2 teaspoon salt. In a large bowl, beat the butter and remaining 1 cup sugar with a mixer on medium speed until light and fluffy, about 3 minutes. Add the eggs one at a time, beating after each addition, then beat in the vanilla. Reduce the mixer speed to low; add the flour mixture in three additions, alternating with the sour milk, beginning and ending with the flour, and knead the dough.
4. Pour the batter over the nectarines and spread evenly. Place the pan in the oven and bake until golden brown and a toothpick inserted into the center comes out clean, about 50 minutes. If the pie is browning too quickly, cover it with foil. Let it rest for 15 minutes, then invert onto a platter and let cool completely. Sprinkle with sea salt.