Preheat the oven to 350°F (175°C).

Add ¼ cup unsalted butter to a 9-inch round cake pan or springform pan. Place the pan in the warming oven just until the butter has completely melted.

Distribute the packed light brown sugar evenly across the melted butter.

Cut the Honeycrisp apples into ¼-inch-thick half-circle slices; peeling is not necessary. Arrange the apple pieces over the brown sugar in overlapping concentric rings until the entire base of the pan is covered. Set the prepared pan aside.

In a large bowl, thoroughly whisk together the all-purpose flour, granulated sugar, baking powder, cinnamon, and salt.

In a separate bowl or measuring vessel, whisk the milk with the cooled melted butter, vanilla extract, and egg until evenly blended.

Pour the liquid mixture into the dry ingredients. Whisk only until the batter becomes smooth and no visible flour streaks remain. Avoid excessive mixing.

Scrape around the sides and bottom of the bowl as needed so the batter is uniformly combined.

Carefully spread the finished cake batter over the prepared apple layer. Use a spatula to level the surface from one edge of the pan to the other.

Transfer the pan to the preheated oven and bake for 30–35 minutes. The cake is ready when the surface appears golden and set and a toothpick inserted into the center comes out clean.

After removing the cake from the oven, gently run a knife around the inside edge of the pan to release the sides.

Let the cake cool completely to room temperature. Position a serving plate over the pan, then carefully invert the cake so the caramelized apple layer becomes the top.