Preheat oven to 350 degrees
Heat butter in a 10" cast iron skillet over medium heat until it's melted
Swish butter around the pan until bottom and sides are coated
Pour melted butter into a large mixing bowl or stand mixer bowl
Keep butter in skillet melted by placing the pan over low heat while you prepare the batter or by placing in the warming oven
Wisk yogurt until smooth and add to mixing bowl
Add vinegar and honey and mix at medium speed for a minute
While mixer is running, add the eggs one at a time
After last egg is added, stop the mixer and add the coconut flour, almond flour, baking soda and salt. Tip: use a wire mesh strainer for the almond flour to sift any lumps out
Mix until just combined
Pour batter into the warm skillet
Bake for 30 minutes until toothpick, or knife, inserted in the middle comes out clean
Cool on wire rack for 15 - 20 minutes before cutting
Store left overs in refrigerator or freeze