Preheat your oven to 350°F. Grease a 12" cast iron skillet with butter or oil.
In a large bowl, whisk together the flour, baking powder, salt, and cinnamon. Set aside.
In the bowl of a stand mixer fitted with a whisk or paddle attachment (or using a hand mixer), beat the eggs on high for 30 seconds. With the mixer still running, slowly pour in the sugar. Continue beating for 3–5 minutes, until the mixture is pale, thick, and tripled in volume.
While the eggs are mixing, cube the butter and place it in a medium saucepan with the apple cider, melted coconut oil, and milk. Warm over low heat until the butter is melted, then remove from the heat and stir in the vanilla.
Turn the mixer to low and slowly add the flour mixture, mixing just until combined. Be careful not to overmix.
Pour in the warm butter mixture and gently mix until the batter is smooth and well combined.
Transfer the batter into the prepared skillet, smoothing out the top. Pop any air bubbles with a spoon or toothpick.
Bake for 30 minutes, or until the cake is golden brown and a toothpick inserted in the center comes out clean.
While the cake bakes, prepare the glaze. In a small bowl, whisk together the powdered sugar, cinnamon, milk, and apple cider until smooth.
Once the cake has cooled slightly, drizzle the glaze over the top and serve warm.
Notes
I often add diced apples to this cake, and it seriously tastes like an apple fritter. Just peel and dice up one apple and add it to the batter as the last step.
You can use all purpose flour, whole wheat, or freshly milled hard white whole grain flour interchangeably in this recipe - no measurement substitutions needed.
For an extra decadent dessert, serve this cake with some vanilla ice cream.
Don't overbake the cake! You don't want any raw or sticky batter on the toothpick, but finding a few crumbs is normal.
This recipe also fits perfectly in a 9x13 baking dish or 2 round cake pans.