Add the butter and chocolate into a medium microwave safe bowl and microwave on high in 15 second increments, stirring in-between, until melted and smooth. Set aside and allow to cool to room temperature.

In a large bowl or the bowl of a stand mixer fitted with the whisk attachment, beat the egg whites on high speed until soft peaks form, about 5 minutes. After that, slowly add 3/4 cup of powdered sugar and continue beating until stiff peaks form. You should be able to hold the bowl upside down without anything falling out.

With the mixer on low speed, gradually add the cocoa powder mixture to the egg white meringue until fully combined, about 1-2 minutes.

Next, gradually fold in the cooled chocolate mixture using a rubber spatula until fully incorporated. Be careful not to overmix and deflate the egg white meringue.

Cover with plastic wrap and place dough in the refrigerator for one hour or up to one day.

Repeat baking process until cookie dough is gone. Serve warm or store in an air-tight container for up to 2 weeks or in the freezer for up to 6 months.