Preheat the oven to 350°F. Position an oven rack in the center.

Generously coat the bottom and sides of an 8-inch round cake pan with 2 tablespoons of softened butter.

A solid cake pan works better than a springform pan because the peaches release juice as they bake. A springform pan may leak the butter, sugar, and peach juices into the oven.

If a springform pan is your only option, wrap the outside tightly with two layers of aluminum foil and place it on a rimmed baking sheet.

In a small bowl, combine ½ cup granulated sugar with the ground cinnamon.

Sprinkle the cinnamon sugar evenly over the buttered bottom of the cake pan. During baking, the sugar will melt together with the butter and peach juices to create a glossy, caramelized topping.

Wash and dry the peaches. Cut them in half, remove the pits, and slice the fruit into thin, even wedges.

Arrange the peach slices in a single layer over the cinnamon sugar. For a decorative presentation, place the slices in concentric circles, slightly overlapping each piece.

Try to cover as much of the bottom of the pan as possible. Once the cake is inverted, this peach arrangement will become the top.

There is no need to peel the peaches. Their skins soften during baking and help the slices hold their shape.

Place ¾ cup softened butter and the remaining ½ cup sugar in a large mixing bowl.

Beat with an electric mixer for 3–4 minutes, until the mixture is pale, light, and fluffy. Stop occasionally to scrape down the sides and bottom of the bowl with a spatula.

Properly creaming the butter and sugar incorporates air into the batter and helps create a softer cake.

Add the egg and vanilla extract. Beat for another 1–2 minutes, until the mixture is smooth and completely combined.

For the best texture, use an egg and milk that have been brought to room temperature. Room-temperature ingredients combine more easily with the softened butter and help prevent the batter from looking curdled.

In a separate bowl, whisk together the all-purpose flour, baking powder, and salt.

Whisking the dry ingredients separately ensures that the baking powder and salt are distributed evenly throughout the cake.

Set the mixer to low speed. Gradually add the flour mixture to the butter mixture in two or three additions.

Pour in the milk and mix just until everything is combined. Add the poppy seeds and gently fold them into the batter with a rubber spatula.

The batter should be thick but spreadable. Avoid overmixing after adding the flour, as too much mixing can make the cake dense or tough.

Carefully spoon the cake batter over the arranged peaches. Add it in small portions so the peach slices remain in place.

Use a spatula to gently spread the batter into an even layer, making sure it reaches the sides of the pan.

Lightly tap the pan on the counter once or twice to help the batter settle into any spaces between the peach slices.

Set the cake pan on a baking sheet and place it in the center of the preheated oven.

Bake for 30–35 minutes, or until the top is golden and a toothpick inserted into the center comes out clean or with a few moist crumbs attached.

If the top begins browning too quickly before the center is fully baked, loosely cover the pan with aluminum foil and continue baking.

Remove the cake from the oven and let it cool in the pan for 10–15 minutes.

Do not let it cool completely before flipping. As the caramelized peach layer cools, it begins to set and may stick to the bottom of the pan.

Run a thin knife around the sides to loosen the cake. Place a large serving plate upside down over the cake pan. Hold the plate and pan firmly together, then invert them in one confident motion.

Carefully lift off the pan. If a peach slice remains stuck to the bottom, gently remove it and place it back on the cake.

Serve the Peach Upside-Down Cake warm or allow it to cool completely before slicing.