Heat the oven to 350 degrees. Spray with baking spray a 10- or 12-cup Bundt pan, making sure to get into all the grooves of the pan.
In the bowl of your stand mixer, cream butter and 1 1/2 cups sugar until pale and fluffy using the paddle attachment (you could also beat it with a hand mixer), 4-5 minutes. Beat in the eggs one by one, waiting until each has been incorporated to add the next. Add the vanilla and mix to combine. Be sure to scrape the mixing bowl after each addition of the eggs.
In large bowl, whisk together the flour, cinnamon, nutmeg, baking powder, baking soda soda, and salt. Set aside.
Add half of the flour mixture to the mixer beat on low speed until incorporated. Scrape down the sides of the bowl to incorporate. Add in buttermilk and beat until incorporated followed by the remaining flour mixture; scrape the sides of the bowl to incorporate.
Spoon the cake batter into the Bundt pan; spread evenly with a spatula. Tap the pan heavily on the counter a few times to help even out the batter and remove air pockets. Bake for 45-55 minutes (ours was done at 45 mins...every oven is different), or until a toothpick inserted into the center of the cake comes out clean and the top springs back when lightly pressed.
Let the cake cool in the pan for 15 to 20 minutes, then flip the pan onto a cooling rack set inside a baking sheet. Tap the pan heavily onto the rack.
In a small bowl, mix the remaining 2/3 cup with the cinnamon to combine. Brush the warm cake all over with melted butter, then spoon cinnamon sugar over the cake. Brush any bare areas with the melted butter and reuse any cinnamon sugar that falls onto the baking sheet below the rack, using your hands to gently press it into the surface of the cake to help it stick. Make sure to fully coat the Bundt with the cinnamon sugar coating. Cool cake completely before serving.