• Line the bottom of a 9-inch round cake pan with parchment paper. Cut out strips of parchment and line the sides as well. (When I didn't do this, my cake stuck a little, although not badly.) Spray with cooking spray. Set aside.
• Cream butter and sugar until light and fluffy with an electric mixer on medium speed. Add eggs, one at a time, and mix until incorporated. Add melted chocolate and vanilla, mixing again until well combined. Add flour and salt and stir just until combined. Stir in nuts, just until mixed in.
• Pour batter into prepared cake pan. Bake at 375F for 25-30 minutes, being careful not to overbake. A toothpick inserted into the center will have a few moist crumbs stuck to it. Cool cake completely.
• If you haven't used parchment, carefully run a knife along the edges to loosen the cake. Carefully turn out cake onto a wire rack, then invert it onto your cake plate.

Frosting

• Beat together 2 teaspoons butter with the sugar and cocoa powder, until butter is completely incorporated. (I was impatient and ended up with flecks of butter in my icing the first time. So I didn't use it.) Pour in 1 ½ tablespoons hot water/coffee. Beat until fluffy. Add more, teaspoon by teaspoon, if necessary. Pour onto cake and let set.