To bake the cake, preheat oven to 350F. Grease a Baking Slab with butter and dust with cocoa powder. Shake out any excess.

Bring ¼ cup cocoa powder, 2 sticks butter, and 1 cup water to a boil in a medium Stainless Clad Saucier or Saucepan, whisking to combine. Remove from heat.

Meanwhile, whisk flour, sugar, baking soda, and salt in a medium bowl. Measure buttermilk in a liquid measuring cup, add the eggs and vanilla, and blend with a fork. Gradually pour buttermilk mixture into flour mixture, whisking to combine. Add cocoa mixture and whisk until smooth. Place Baking Slab on a Sheet Pan. Scrape batter into Slab; do not overfill. Smooth top with a Spatula.

Bake until cake starts to pull away from sides of Baking Slab, the center is set and springs back to the touch, and a tester inserted into the center comes out clean, 35–40 minutes.

To top the cake, heat a large Stainless Clad Frying Pan over medium. Add pecans and toast, shaking pan occasionally, until aromatic and lightly browned, about 5 minutes. Make sure the pecans are toasted before you make the frosting

To make the frosting, during the last 10 minutes or so of baking the cake, heat the butter, milk, and cocoa powder in a medium Saucier or Saucepan over low heat. Whisk occasionally until butter melts and mixture is smooth but not boiling. Remove from heat and whisk in powdered sugar, vanilla, and salt.

Pour warm frosting (make sure it’s still warm so it can spread more easily) over warm cake and smooth with a Spatula. Top with pecans and sprinkle with flaky salt. Slice and serve warm or at room temperature.