Preheat oven to 350 degrees F. Grease an 8" metal pan
with butter. In a large bowl whisk egg yolks and sugar until thick.

In a saucepan over low heat, melt butter. Add chocolate and
melt, being careful not to overheat. Remove from heat and let cool until it is
lukewarm. (You can also use a microwave-safe bowl and heat on high for 1 minute
in the microwave).

Add chocolate mixture to the egg and sugar mixture and mix
well. Sift in almond flour, coconut flour, and espresso powder. Mix together.

Using a mixer, mix egg whites until they form soft peaks.
Carefully fold egg whites into the chocolate batter, trying to maintain as much
air as possible in the batter. Pour into the pan.

Bake for 25-28 minutes. The brownies will be set but will
still slightly wobble in the center. Leave brownies in the pan and set on a
rack to cool. Once they have completely cooled you can slice and remove from
the pan.