Preheat oven to 350 degrees F. Use coconut oil (or butter) to grease an 8″ cast iron skillet, set aside.
In a bowl attached to a stand mixer (or with a hand mixer), cream together butter, zest, and sugar until creamy, several minutes. Add in egg, and vanilla and mix thoroughly, another couple minutes.
Remove bowl from mixer and by hand, add in half of the dry ingredients. Add buttermilk, stir, then add rest of dry ingredients and mix until just combined. Stir in blueberries.
Pour batter into prepared pan, smoothing evenly with a spreader. Sprinkle with reserved sugar. Bake for 40-45 minutes or until toothpick inserted in middle comes out clean. Let cool slightly before slicing.
