Sanitize all work surfaces, tools, and hands before handling raw flour and batter.
Preheat oven to 350°F.Grease a half-sheet pan (18-in x 13-in x 1-in).
Melt together unsweetened chocolate and unsalted butter in a double boiler. Remove from heat and maintain the temperature of the chocolate/butter mixture at 86 degrees F°.
Meanwhile, combine whole eggs and sugar in a 5-quart mixer bowl. Using the paddle attachment, beat on low speed (Speed 1 on a Hobart mixer) for 1 minute. Stop the mixer and scrape down the bowl.
Add the chocolate and butter mixture. Beat on low speed for 1 minute. Stop the mixer and scrape down the bowl. Stir in pure vanilla extract.
Add sifted flour. Beat on low speed 1 minute. Stop the mixer and scrape down the bowl.
Pour the batter into the prepared sheet pan and spread out evenly. Bake in preheated oven for 40 minutes. A wooden pick inserted in the center will come out with moist crumbs (150°F).
Sanitize all work surfaces, tools, and hands after handling raw flour and batter.
Cool to room temperature. Double-wrap slab of brownies in a sheet pan with cling wrap and refrigerate at least 4 hours or overnight.
Using a long sharp chef’s knife, trim off 1/2-in from each long side of the slab. Cut the slab into twenty 3-in x 3-in brownies.