Preheat the oven to 325°F. Set a 10-inch cast-iron skillet over low heat. Melt the butter. Rotate and tilt the pan so a bit of the butter goes up the sides to grease it. Turn off the heat, then add the honey, cinnamon and brown sugar to the pan.
Whisk in the pan until smooth before sprinkling the hazelnuts and seeds evenly over the honey mixture in the skillet. In a bowl of an electric mixer, cream the butter and sugar together until light and fluffy. Add eggs, one at a time, and then the vanilla.
Whisk the dry ingredients together in a separate bowl.
Mixing on low, alternate adding the dry ingredients and milk to the batter. Take care to scrape the sides and bottom of the bowl to make sure everything is incorporated.
Pour the cake batter over the nut mixture in the skillet, making a spiral from the outside in. Gently spread the batter evenly. Do not stir!
Bake for 55–60 minutes. Check for doneness with a toothpick in the center. As soon as you take the cake from the oven—the honey nut mixture is still piping hot, and will move!—immediately invert the cake onto a flat plate or rack. Serve warm or at room temperature.
Wrap tightly to store.