Preheat the oven to 325ºF (170ºC).
Butter or spray an 8-inch square baking pan and line it with parchment paper. I use a strip wide as the pan and line the bottom and 2 of the sides.
Melt 10 tablespoons unsalted butter in a large mixing bowl, preferably glass, in the microwave or over simmering water.
Add 1 ¼ cups granulated sugar, 14 tablespoons unsweetened cocoa powder, sifted, ¼ teaspoon salt and ½ teaspoon vanilla extract, and stir with a whisk until well mixed.
Add 2 cold eggs, one at a time, and whisk until mixed well.
Add ½ cup all-purpose flour and whisk vigorously until the batter is shiny and smooth.
Bake for 20 to 25 minutes until a cake tester or toothpick comes out with moist crumbs attached, not wet. The center must jiggle. Don't overbake them. If in doubt, take them out a few minutes sooner than later.
Let cool completely on a wire rack.
Cover with a layer of chocolate ganache. Let harden before cutting.