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1
Preheat oven to 350 F. Grease a 9x13 pan with butter, line with parchment paper and butter the parchment. Or if you prefer you can just butter the pan and dust with cocoa powder.
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2
For the brownies layer:
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3
Place butter and chocolate in a small saucepan over low heat and heat until melted.
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4
In the bowl of a stand mixer fitted with the whisk attachment add eggs and sugars. Whisk until pale yellow and fluffy, about 5 minutes on medium. Add melted chocolate and vanilla to the egg mixture. Mix until almost incorporated.
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5
Add dry ingredients (flour through salt) into the eggs and mix until fully incorporated on stir setting, scraping down sides of the bowl as needed. Add sour cream and nuts and stir them in using a rubber spatula until fully incorporated.
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6
Spread brownie layer evenly over the bottom of greased pan.
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7
For the Brown Butter Chocolate Chip Cookie Layer:
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8
In a small saucepan add 10 tablespoons butter and melt over medium heat until butter is brown and nutty smelling. Swirl the pan as needed to encourage butter to brown evenly. Remove from heat.
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9
Add browned butter into a mixing bowl and add remaining 4 tablespoons cold butter that you've cut into 1 tablespoon sized pieces. Mix until butter is melted.
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10
Add both sugars, vanilla and sea salt. Mix until blended.
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11
In another small bowl whisk whole egg with egg yolk and add it into the butter/sugar mixture. Beat on medium high speed 30 seconds. Let mixture rest 3 minutes.
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12
Repeat beating mixture for 30 seconds and resting for 3 minutes until mixture becomes thick and pale. Add baking soda and mix for 30 seconds. Scrape down the sides of bowl. Add flour and mix on low speed just until combined. Stir in chocolate chips.
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13
Drop the cookie dough into the pan doing it evenly by spoonfuls over the top of brownie layer.
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14
Bake for 35-45 minutes. The 35 minutes are for a gooier brookie. Brown Butter Brookies are done when chocolate chip cookie layer is light brown and brownies have risen around chocolate chip cookies. Remove pan from oven and set it on a rack. Allow bars to cool for 1 hour before cutting.
