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1
Butter a foil-lined pan and set aside. Preheat oven to 375u00b0 F.
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2
Chop walnuts into large chunks and set aside. Sift together the flour, cocoa, salt, and espresso powder and set aside.
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3
Place the butter and unsweetened chocolate in the top of a double boiler over medium heat and cook until the chocolate and butter are melted. Stir until smooth and set aside.
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4
Using a stand mixer, beat the eggs to mix. Carefully add the sugar and beat together for 15 minutes. Add the vanilla and sour cream to the egg mixture and mix briefly.
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5
Add the chocolate mixture, then the flour mixture and beat on low speed, scraping the sides of the bowl a few times just until thoroughly combined. Don't over-mix!
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6
Remove bowl and fold in the walnuts by hand. Pour into the prepared pan, then smooth the top using a small offset spatula. Bake for 20 to 25 minutes, or until a tester inserted in the middle comes out clean.
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7
Remove the pan from the oven and let stand 5 minutes. Cover the entire pan with a cooling rack and invert the pan and rack. The pan should lift off, along with the foil. Cool the slab completely and carefully wrap it in plastic wrap and foil, then freeze for up to two months.
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8
To make the espresso ganache, place the chocolate into heatproof bowl. In a small saucepan, heat the cream until bubbles appear. Stir in the espresso powder until dissolved.
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9
Immediately pour the hot cream mixture over the chocolate and let stand for 1 minute. Stir the chocolate until melted and smooth. Let the ganache stand at room temperature several hours until cool and firm enough to spread.
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10
Take frozen brownie slab out of freezer. Place on a cutting board. Slather with ganache and sprinkle with sea salt. Cut into small bites. Any leftovers (as if!) can be frozen.
