Preheat oven to 350°F. Line an 8 x 8 inch square pan with parchment paper. To make it easier to remove the brownies later, have an overhang of parchment paper over the edges of the baking pan. Set aside when done.
Place the butter in a heat-proof bowl and microwave to completely melt the butter.
Add the brown sugar, white sugar and salt while the butter is still warm. Mix to combine the butter and sugars together until you have a smooth paste.
Add the vanilla, coffee extract (if using), eggs and egg yolks to the butter-sugar mix. Whisk in the eggs until they have been incorporated into the butter sugar mix. Using a hand mixer, whisk the butter-sugar-egg mix on medium speed, until the batter becomes pale in color (about 2 minutes).
In a separate bowl, sift the flour and cocoa powder together.
Add the flour-cocoa mix into the wet ingredients (in two batches if that’s easier) and mix it in until just combined. DO NOT OVER-MIX. Add about ? of the chopped chocolate halfway through the mixing in of the flour. The goal is to make sure the batter is not over-mixed when the flour is being incorporated.
Scrape the batter into the prepared pan. Spread the batter evenly. Sprinkle the rest of the chocolate on top.
Bake for 30 – 40 minutes (30 minutes = extra fudgy, 35 minutes = perfectly fudgy for my liking, 40 minutes = less fudgy).
Remove from the oven and let it cool down to room temperature. Remove the brownies from the pan and let them chill in the fridge for a couple of hours (chilled brownies are easier to cut).
Cut the brownies into 16 pieces. Use a warm knife to cut, making sure to clean the blade after each cut for clean, neat edges.
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