Step 1

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.

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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.

Step 6

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.

Step 7

Immediately pour the warm frosting over the warm cake and gently spread to cover. Let the cake cool completely before slicing.