Preheat the oven. Preheat the oven to 350°F (175°C).

Melt the topping butter. Place 1/4 cup unsalted butter in a 9-inch round cake pan. Put the pan in the oven for a few minutes, just until the butter melts. Carefully remove it from the oven.

Add the brown sugar. Sprinkle the light brown sugar evenly over the melted butter, covering the bottom of the pan.

Prepare the apples. Core the Honeycrisp apples and cut them into half-circle slices approximately 1/4-inch thick. Peeling is optional.

Arrange the apples. Place the apple slices over the brown sugar in concentric circles, slightly overlapping each piece. Continue until the bottom of the pan is completely covered.

Combine the dry ingredients. In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, cinnamon, and salt.

Add the wet ingredients. Pour in the milk and 1/4 cup melted butter, then add the vanilla extract and egg. Whisk gently until just combined. Avoid overmixing.

Add the batter. Carefully pour the cake batter over the arranged apples. Gently spread it from edge to edge without disturbing the fruit.

Bake. Bake the Apple Upside Down Cake for 30–35 minutes, or until a toothpick inserted into the center comes out clean or with only a few moist crumbs.

Cool. Run a thin knife around the edge of the cake to loosen it. Allow the cake to cool before turning it out.

Flip the cake. Place a large serving plate upside down over the cake pan. Holding the plate and pan firmly together, carefully flip them. Slowly lift away the pan to reveal the caramelized apple topping.