Heat the oven to 375°F. Cut the sweet potatoes in half lengthwise and roast them cut side down on a lined baking sheet for 30 to 40 minutes, until very soft.
2 cups sweet potato puree
Lower the oven to 350°F. Line your baking dish with parchment paper.
Scoop the sweet potato out of the skins into a large bowl and mash until smooth. Measure 2 cups.
Add the sugar, eggs, vanilla bean paste and espresso. Mix well.
1 cup sugar, 2 large eggs, 2 teaspoons vanilla bean paste, 1 shot espresso
Add the flour, cocoa powder, baking powder and salt. Mix until just combined, then fold in the chocolate chips.
1 cup unbleached all-purpose flour, ½ cup cocoa powder, 1 teaspoon baking powder, ½ teaspoon salt, 1 cup chocolate chips
Spread the batter into the dish. Bake at 350°F for 35 minutes, until a toothpick in the center comes out clean or with a few moist crumbs. If it's still wet, bake 5 more minutes.
Cool in the dish for at least 15 minutes before slicing.
