Preheat oven to 350°F degrees. Butter or spray an 8x8 baking pan, set aside
In a mixing bowl, combine flour, sugar, and salt.
In a saucepan, melt the butter. Add the cocoa and stir together. Add hot water and allow the mixture to boil for 30 seconds and then turn off the heat. Pour over the flour mixture and stir with a wooden spoon or rubber spatula.
Whisk the milk, egg, baking soda, and vanilla. Add the milk mixture into the batter and stir with a wooden spoon or rubber spatula until combined. Pour into prepared pan and bake at for 15-20 minutes, until a toothpick inserted into the center comes out clean. Upon removing the cake form the oven immediately prepare the icing.
Icing:
Melt the butter in a saucepan. Add the cocoa, stir to combine, then turn off heat. Add the milk, vanilla, and powdered sugar. Whisk until smooth and combined. Stir in the nuts. Pour over the warm cake and gently spread into an even layer. Allow the cake to cool and the icing to set before serving.
Notes
*Toasting nuts: Place nuts in a dry skillet over medium heat. Heat the nuts, stirring occasionally, until toasted and fragrant. Serving: Serve at room temperature. If desired, you can sprinkle the top of each serving with flaky sea salt to balance out the sweetness. Storage: This cake can be stored tightly covered at cool room temperature for up to 3 days. You can freeze leftovers for up to 2 months.Recipe adapted from The Pioneer Woman