Prepare the Oven and Cake Pan: Preheat the oven to 180°C (350°F). Line the bottom of a 20 cm (8-inch) round cake pan that is at least 6 cm (2.5 inches) tall with parchment paper. A pan approximately 7 cm (3 inches) tall also works. Alternatively, use a 23 cm (9-inch) pan for a shorter cake. Coat the Pan with Vegan Butter: Pour the 55 g of melted vegan butter for the topping into the prepared cake pan. Gently tilt and rotate the pan to spread the butter across the bottom and up the sides.
Add the Topping Sugar: Sprinkle 50 g of granulated sugar evenly over the melted butter in the bottom of the pan.
Arrange the Peach Slices: Place the thinly sliced peaches over the butter and sugar mixture. Arrange them in overlapping concentric circles or another decorative pattern, covering as much of the base as possible to avoid gaps. Set the pan aside. Combine the Dry Ingredients: In a medium-sized mixing bowl, add the all-purpose flour, 200 g of granulated sugar, baking powder, and salt if using unsalted vegan butter. Whisk until the ingredients are evenly distributed.
Mix the Cake Batter: Add the room-temperature dairy-free milk, neutral-flavored oil, melted vegan butter (or extra oil), vanilla extract, and optional lemon juice or apple cider vinegar and ground nutmeg or cinnamon. Stir until the ingredients are just combined. Avoid mixing more than necessary. Fill the Cake Pan: Carefully pour the batter over the arranged peach slices. Spread it gently if needed so the batter covers the fruit evenly.
Bake the Cake: Place the pan in the preheated oven and bake for approximately 47–52 minutes for a 20 cm (8-inch) pan. If using a 23 cm (9-inch) pan, bake for approximately 43–48 minutes. If the top begins browning too quickly, carefully cover the pan loosely with aluminum foil. Check for Doneness: Insert a toothpick into the center of the cake. It is ready when the toothpick comes out mostly clean; a few moist crumbs are acceptable.
Cool the Cake Briefly: Remove the cake from the oven and place the pan on a wire rack. Allow it to cool in the pan for 10 minutes before attempting to invert it.
Invert onto a Serving Plate: Place a serving platter over the top of the cake pan. Holding the pan and platter securely, carefully and quickly turn them upside down so the cake releases onto the platter with the peach topping facing upward. Cool and Serve: Let the cake cool until it has mostly reached room temperature, or serve it at room temperature. Pair each slice with dairy-free vanilla ice cream or a dollop of whipped dairy-free cream, if desired. Store Leftovers: Place leftover cake in an airtight container and refrigerate for up to 3 days. Allow individual slices to return to room temperature before serving. Leftover slices may also be frozen, although the peach topping may change in texture and appearance after freezing.