Place a piece of parchment in an 8-inch cake pan. Grease the parchment and sides of cake pan with coconut oil.
In a large bowl, using an electric hand mixer on medium speed, cream the ½ cup unsalted butter and 1 cup granulated sugar, about 3 minutes. Add the 2 large eggs one at a time, beating until combined after each and scraping down the sides and bottom of the bowl as needed. Beat in the 1 teaspoon pure vanilla extract.
In a medium bowl, whisk together the 1 ¾ cups gluten-free flour, 1 ½ teaspoons baking powder, and ½ teaspoon fine sea salt.
With the mixer on low, stir half of the flour mixture into the butter mixture until just combined. Stir in the ½ cup whole milk until just combined, then add the rest of the flour, scraping down the sides and bottom of the bowl as needed.
Position a rack in the center of the oven and heat to 350º F.
Butter an 8 or 9-inch cake pan and line it with a circle of parchment paper. Rub the paper with 1-2 tablespoons of butter.
Arrange the 2 large nectarines slices in the pan in concentric circles, having them slightly overlap each other if you can. Drizzle on the 1 tablespoon maple syrup and sprinkle with 1 teaspoon ground cinnamon.
Spread the batter evenly over the fruit in the pan, then rest the batter for 15 minutes.
Place the cake in the oven and bake until the top is golden and a toothpick inserted near the center comes out clean, the top is set, and the edges are slightly coming away from the pan, 40-50 minutes. Remove from oven and let cake cool completely in the pan on a rack, then invert onto a serving platter and peel away the parchment. Serve the cake at room temperature.
Notes
CN's tips
What size pan? This depends on your preferred cake-to-fruit ratio. I like a thickercake, so I typically use an 8-inch cake round. However, for a thinner cake, use a 9-inch cake round.
Add almond flour - To add a little extra moistness to the cake, substitute ½ cup of almond flour for ½ cup of the gluten-free flour.
Almond extract - I like to stir in ½ teaspoon of almond extract for a subtle almond flavor note.
Dairy-free - Make this cake dairy-free by using non-dairy butter and non-dairy milk.
Spices - I've made this cake using ground ginger, cinnamon, and cardamom, and I heartily endorse any of these spices! You could also do a blend. The ground spice adds a bit of warmth. Conversely, you could leave off the spice if you wish for a plainer cake.
Try peaches - For a peachy upside-down cake, try sliced peaches instead of nectarines!
Or blueberries - And/or, add 1 cup of blueberries to the nectarines for a stone fruit-and-berry cake.