Prepare a 9-inch round cake pan by spraying the bottom and sides with a light coating of cooking spray. Set aside.

In a small, heavy-bottomed pot over medium heat, melt 1/4 cup unsalted butter. Once the butter is just about melted, mix in the thyme, 1/2 cup dark brown sugar, and 1/2 teaspoon kosher salt. Continue cooking just until the sugar begins to bubble, about 1 minute. Pour the mixture into a 9-inch round cake pan and swirl around until the mixture spreads evenly across the bottom of the pan.

Arrange the orange slices on top of the brown sugar mixture. Make sure you fully cover the bottom of the pan — this will lead to a prettier cake! Don't be scared to overlap any spaces with smaller slices here and there. Once you've used up all the orange slices, set aside and prepare the cake batter.

Center a rack in the oven and preheat to 350 (F).

In a medium bowl, whisk together 2 cups all-purpose flour, 1 3/4 teaspoons baking powder, and 1/2 teaspoon kosher salt until well combined. Set aside.

Once the mixture is light and fluffy, lower the mixer speed to its slowest setting and add 2 large eggs one at a time, only adding the second egg once the first is fully incorporated. Add 1 teaspoon pure vanilla extract.

Once the vanilla extract is incorporated, keep your mixer on the lowest setting and add half the flour mixture (from the 2nd step). Once the flour is fully incorporated, add 3/4 cup whole milk. Continuing mixing for about a minute or so, just until the mixture looks like it's completely moist, then add the remaining half of the flour mixture. Mix just until the flour streaks disappear into the batter — at this point, the batter will probably look like it's curdled. But don't worry! It's supposed to look that way. It'll come together in the oven, I promise.