Preheat the oven to 350℉/175℃. Line an 8x8 baking pan with parchment paper or foil. I recommend having it overhang on the sides so it's easier to remove the brownies once cool.
Over medium-low heat, melt the butter in a small saucepan. Once melted, the butter will begin to foam and sputter. You'll see little white dairy solids at the bottom of the pan. Keep cooking and stirring with a spatula or wooden spoon until the milk solids begin to brown and it smells nutty and toasted.
¾ cup unsalted butter
Pour the brown butter into a large microwave safe bowl and let cool slightly.
Add the brown sugar and granulated sugar. Whisk until smooth and place back in the microwave for 20 -30 seconds. Whisk again for 1-2 minutes.
1 cup granulated sugar, ½ cup light brown sugar
Whisk in the salt and vanilla extract and cocoa powder. Microwave for 20-30 seconds. Whisk for about a minute. At this point it’s going to look weird and grainy and that’s totally normal!
1 teaspoon vanilla extract, 1 teaspoon kosher salt, ½ cup dutch process cocoa powder
Add the eggs and egg yolk. Finish with 2-3 minutes of whisking - sorry if your arm is getting sore!
2 large eggs, 1 large egg yolk
Stir in the flour and espresso powder. It’ll be a shiny fairly thick brownie batter.
½ cup all purpose flour, ½ tablespoon espresso powder
Fold in the chocolate shards with a rubber spatula. You want them to be chopped quite finely so they'll easily melt in the brownies when baking.
4 oz semisweet chocolate
Pour into prepared 8x8 pan. Bake for 22-30 minutes. You should have a crinkle top and if you stick a toothpick in, it'll come out with some wet crumbs but not raw batter.
Let the brownies cool completely in the pan before removing and slicing up. I recommend chilling the brownies in the fridge for a few hours if you want them to be extra fudgy!
Video
Notes
This recipe is adapted from Buttermilk by Sam's Crinkly Brownies, thanks to Sam for sharing her findings and method! My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. I have provided a conversion to US cups but please note that I haven’t tested the recipe with this method. Just click on ‘Metric’ above the ingredients to see the metric measurements.Don't have a microwave? If you don't have a microwave you can use the double boiler method instead. Fill a large saucepan with an inch or two of water and bring the water to a simmer. Place your large bowl on top, making sure the water doesn't touch the bowl. Follow steps until you've added the cocoa powder, then remove the bowl from the saucepan. Kosher Salt: if you don’t have kosher salt, halve the amount of salt in the ingredient list (1 teaspoon kosher salt = ½ teaspoon fine salt). Please note, that I'm using Diamond Crystal Kosher Salt. Morton Salt is a different type of kosher salt and is much saltier. Cocoa Powder: I'm using dutch processed cocoa powder but you can swap it out for natural cocoa powder, just note that the brownies won't be as rich. Espresso Powder: use a fine espresso powder and not espresso granules. Pan: I recommend using a metal pan and not a glass pan. A glass pan can't conduct heat like metal can, so your brownies will have overcooked sides and bottom but an undercooked metal. Storage: Keep them in an airtight container in the fridge for up to 5 days.