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Line a 9x9 square pan with parchment paper (or grease it well). Set aside.
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Melt the unsalted butter and semi-sweet chocolate together until combined. (You can do this on the stove or in the microwave.)
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Add in the granulated sugar and brown sugar, and mix until combined. Allow the mixture to cool about 10 minutes.
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Next, add in the eggs, one at a time. Whisk well after each addition.
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Add in the flour, cocoa powder, espresso powder, and salt. Mix until combined.
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Add in the vanilla extract and mix until combined.
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Lastly, add in the toasted pecans, and mix until combined.
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Pour the chocolate fudge cake batter into the prepared baking dish and bake at 300 degrees for 45-50 minutes, or until a cake tester comes out clean (a few fudgy crumbs stuck to the cake tester is fine).
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Allow to cool at least 45-60 minutes before serving. Dust with powdered sugar or cocoa powder, if desired.