Combine sugar, all-purpose flour, almond flour and cocoa powder in a medium bowl and whisk to distribute evenly.
Add the egg white and whisk to incorporate the dry ingredients completely. Scrape the side of the bowl as necessary. Set aside.
For the brown butter, start with 5 ounces (1 1/4 stick) of unsalted butter, heat in a small sauce pan over medium heat. When the butter has melted, start whisking. The butter will start bubbling rapidly. When the bubble subsides, the butter will start to brown, continue whisking. When the butter smells nutty and becomes caramel in color, it is done. Take off the heat.
Pour the brown butter in a heat proof bowl, discard any burned milk solid at the bottom of the pan.
Wipe the pan clean with a piece of paper towel.
Measure out 1/2 cup of brown butter, reserving anything extra for later, and add it back into the sauce pan along with the chocolate. If the chocolate doesn’t melt completely in the hot butter, melt over medium heat and whisk to combine.
Quickly pour the melted chocolate butter mixture into the medium bowl with the batter you set aside earlier and whisk to combine.
Place the batter in the fridge for about 1 hour.
Brush 20 cavities of a mini muffin pan with the left over brown butter, and sprinkle with flour.
Preheat oven to 425°F.
Remove batter from the refrigerator and transfer to a piping bag or a ziploc bag. Cut the tip off, and pipe the batter into the prepared muffin pan, filling it to just below the rim.
Reduce oven temperature to 350°F and bake for 14 minutes.
A toothpick inserted in the center should come out clean when done.
Place a clean towel on your counter and flip the muffin pan over to unmold the financiers. If any of them refuse to come out, loosen it with a butter knife.
Let them cool completely on a cooling rack.