This cake is a chocolate lovers dream! A tender chocolate sheet cake is covered in a warm chocolatey, pecan frosting, making this an easy dessert that comes together in a flash. Slice into squares and serve at large family gatherings year-round.
For the cake: Preheat the oven to 350 degrees F; butter the bottom and sides of a 15 1/2-by-10 1/2-inch jelly roll pan.
Whisk together the flour, granulated sugar, baking soda, baking powder and salt in a large bowl until combined. Whisk the sour cream, vanilla and eggs in a medium bowl or large liquid measuring cup until smooth. Set aside.
Meanwhile, add the butter, cocoa powder and 1 cup water to a medium saucepan and set over medium heat. Cook, stirring occasionally, until the butter is completely melted, about 5 minutes.
Pour the melted butter mixture into the flour mixture and stir until combined. Stir in the sour cream mixture in 2 additions until combined. Pour the batter into the prepared pan and spread it into an even layer.
Bake until a toothpick inserted into the center comes out clean, about 20 minutes. Transfer to a rack and let cool slightly in the pan.
For the frosting: Combine the pecans, butter and salt in a medium saucepan and place over medium heat. Cook until the butter is melted and the pecans are lightly toasted, about 5 minutes. Stir in the cocoa powder and vanilla and remove from the heat. Whisk in the confectioners' sugar in 2 batches, alternating with the milk and mixing well after each addition.
Immediately pour the warm frosting over the warm cake and gently spread to cover. Let the cake cool completely before slicing.
This cake can also be made in a 9-by-13-inch baking dish; bake until a toothpick inserted in the center comes out clean, about 30 minutes.