Line a 9x9 square pan with parchment paper (or grease it well). Set aside.
Melt the unsalted butter and semi-sweet chocolate together until combined. (You can do this on the stove or in the microwave.)
Add in the granulated sugar and brown sugar, and mix until combined. Allow the mixture to cool about 10 minutes.
Next, add in the eggs, one at a time. Whisk well after each addition.
Add in the flour, cocoa powder, espresso powder, and salt. Mix until combined.
Add in the vanilla extract and mix until combined.
Lastly, add in the toasted pecans, and mix until combined.
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).
Allow to cool at least 45-60 minutes before serving. Dust with powdered sugar or cocoa powder, if desired.