Preheat oven. Coat an 8-inch baking pan with non-stick spray.
Melt the chocolate, butter, cocoa and espresso powder, either in a microwave or over low heat in a saucepan. In a microwave, do it on high for about 30 seconds each time then stir. When the chocolate is nearly all melted, stop and keep stirring until it’s smooth.
Stir in the sugar, vanilla and salt. Add the eggs one at a time, blending well after each addition.
Fold in the flour and walnuts. Spread batter in the prepared pan and bake until a toothpick inserted in the center comes out clean; about 30 minutes.
Cool completely then cut into squares.
To toast nuts, spread them in a single layer on a baking sheet in a preheated 350 degree F oven for 5-10 minutes.