Melted the butter in the microwave and set aside.
Whisk together coconut flour, buckwheat flour, corn starch, baking powder, and salt in a small bowl.
In a medium bowl beat the eggs with the sugar and vanilla extract at high speed for 3 minutes.
Add the melted butter and the milk and beat a little more.
Gradually add the flour to the wet ingredients and mix well with a rubber spatula.
Let chill the dough for at least 1 hour in the fridge.
Preheat the oven to 425°F (220°C). Spray or lightly oil a madeleine pan.
Pour the mixture and fill the madeleine pan ¾ full, dividing the mixture as evenly as possible.
Cooked for 10/12 minutes or until golden brown. After 4 minutes lower the temperature to 350°F. Be careful because it is very easy for the madeleines to burn, they have to be barely golden.
Let cool before removing.