***THIN, CHEWY VERSION RECIPE***
1 stick (1/2 cup, 113g) butter
1/2 cup (50g) cocoa powder (l like dutch)
1 14 oz (396g) can sweetened condensed milk
1 cup (120g) powdered sugar
2 teaspoons vanilla
2 teaspoons molasses (optional; if you're not getting good skin, try reducing or eliminating this) pinch of salt (only if you used unsalted butter; I put in about 1/4 teaspoon of Morton Kosher)
1 egg yolk
3/4 cup (90g) all-purpose flour
1 cup chocolate chips (optional, I prefer this recipe without them)
Get your oven heating to 325ºF, 160ºC. This batter is very sticky, so I suggest lining your pan with a parchment sling. I use a 9-inch (23 cm) square pan.
Put the butter in a mid-sized pot and heat over medium until it melts and the water boils out. Stir the butter as it starts to brown, to keep the milk solids from sticking to the bottom. When the butter is just browning and smelling like caramel, whisk in the cocoa powder. Let the cocoa boil for a minute then take the pot off the heat.
Whisk in the sweetened condensed milk, then the powdered sugar until it's fully dissolved. (You'll still see the milk solids in the mixture, which won't appear to dissolve. They're fine.) Whisk in the salt, vanilla, molasses (I just drizzle the molasses straight out of the bottle — too sticky to measure), and egg yolk. Whisk in the flour. If you want to stir in solid chocolate chips, let the batter cool down a bit first, or they'll instantly melt.
Get the batter in the pan — don't worry if it's not in an even layer. Bake 30-40 min, until a toothpick to the center comes out clean, or not-quite-clean if you prefer fudgy brownies. Let cool thoroughly before cutting.