Beat the eggs together until fluffy and light yellow.  A large whisk or mixer on medium speed for about 1 minute works well for this.  Brownies have no baking powder, so they rely on thoroughly whipping the eggs for a bit of lift.

Once the eggs are whipped, whisk in the sugar, butter, and vanilla.

If you're feeling lazy, you can just dump the rest of the dry ingredients in and whisk until just combined.  For better brownies, whisk the cocoa powder and flour together in a separate bowl before adding the mixture (or ideally, sifting it into the wet ingredients).  The main reason to mix the cocoa and flour is to break up any lumps, and ensure that the whole mixture is cocoa colored (brown).  Any remaining lumps in the batter won't be visible, but if you just dump the flour in, even a single lump makes a bright white eyesore in the brownies.

Melt 1 tablespoon of butter in a cast iron skillet and heat until very hot but not smoking.  Quickly pour the batter into the pan and spread it evenly.  Top with nuts or a topping of your choice.

Bake at 300 degrees for 45 minutes, or until the center is just barely firm.