½ pound (2 sticks) unsalted butter, melted and cooled to room temperature, plus more for the pan
2 cups all-purpose flour, plus more for the pan
1 tsp. baking powder
½ tsp. baking soda
½ tsp. ground cinnamon
½ tsp. kosher salt
1¼ cups pure maple syrup
½ cup whole milk, at room temperature
3 eggs, at room temperature
1 tsp. vanilla extract
3 Granny Smith apples, peeled, cored, and thinly sliced
½ cup walnuts, toasted and roughly chopped
Active time: 30 minutes Total time: 1 3/4 hours
Preheat oven to 350°. Lightly grease a 9-inch round, 2-inch deep cake pan with butter, line the bottom with a round of parchment paper, and lightly grease again. Dust with flour, tapping out any excess.
In a medium bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt. In a large bowl, whisk together melted butter, syrup, milk, eggs, and vanilla, then whisk in flour mixture until just combined. Stir in apples and walnuts until well incorporated, then transfer to prepared pan and spread out evenly. If you'd like, arrange some of the apples in the batter into a decorative spiral pattern on top.
Bake until cake is deep golden brown and springs back in the middle when touched, 60 to 70 minutes. Set aside until cool enough to handle. Loosen edges with a knife and transfer to a large plate, discarding parchment paper. Serve warm or at room temperature.