Step 1

Make the cake:

Step 2

Preheat the oven to 375 F. Line a jelly roll pan (10½- by 15½-inch rimmed baking sheet) with parchment paper and spray with nonstick cooking spray.

Step 3

Sift the flour into a large bowl. Add the granulated sugar, baking soda and salt. Whisk to combine.

Step 4

In a medium saucepan, combine the butter, cocoa powder and 3/4 cup water over medium heat. Whisk constantly until it begins to simmer, a minute or so. Pour into the dry ingredients and whisk until slightly combined, but not totally. In a small bowl, whisk together the buttermilk, eggs and vanilla until just combined. Stir into the flour mixture and gently fold to combine using a rubber spatula. Transfer to the prepared pan.

Step 5

Bake until puffed and golden and a cake tester comes out clean, 15 to 17 minutes.

Step 6

Make the icing:

Step 7

Meanwhile, in a medium saucepan (I use the same one) over medium heat, combine the butter and cocoa powder, whisking constantly until it begins to simmer. Remove the pan from the heat. While whisking, slowly stream in the milk and vanilla. Add to the large bowl with the powdered sugar and whisk quickly to combine. Stir in the pecans. Do not let cool; pour directly onto the hot cake, straight from the oven.

Step 8

To assemble:

Step 9

Let the cake cool in the pan. Cut into squares and serve with vanilla ice cream, if desired.