• Preheat oven to 350 degrees (F).
• Melt the butter in a small pot over medium heat until it's just starting to brown and give off a nutty aroma, about 8 minutes Stir in coconut oil and then cool the butter mixture to room temperature while you're preparing your other ingredients.
• Generously grease the madeleine mold - making sure you get in all the little ridges - and dust with flour, tapping out any excess flour that may be easily removed. You can also use a cooking spray that has flour in it to make things easier.
• Place the eggs and salt in the body of an electric mixer fitted with the whisk attachment. Whip on high speed until VERY thick - you're looking for the eggs to double or triple in volume - approximately 3 minutes. Continuing to mix on high speed, slowly add the sugar in a steady stream. Whip for another 3 minutes or until mixture is thick and almost ribbony. Fold in the lime zest and vanilla, mixing just until combined.
• Sprinkle the flour on top of the egg batter, and gently fold in. Now fold in the butter/oil mixture. Only stirring enough to bring everything together.
• Pour the batter into a large spouted measuring cup, then pour the batter into the molds, filling each mold 2/3 full.
• Bake the madeleines for about 10 minutes, or until the edges of the madeleines are golden brown.
• Remove from oven and unmold immediately. Cool on a wire wrack completely.

For Chocolate Coating and Assembly:

• Melt chocolate and coconut oil in a double boiler or in a small sauce pan over VERY low heat until chocolate is completely melted and can be whisked smooth; whisk mixture until VERY smooth.
• Dip the bottom of each madeleine into the chocolate, then gently dip in toasted coconut if using. Transfer cookies back to the cooling wrack once dipped. After all the cookies have been dipped, transfer the cooling wrack to set for at least 30 minutes, or up to 4 hours.
• Let cookies come to temperature for at least 20 minutes before serving.