Preheat the oven to 375ºF.

Grease an 8-inch square baking pan with a little butter, then cut a square of wax paper to fit neatly in the bottom.

Melt the chocolate and butter in a glass bowl over a pan of simmering water. Remove from the heat and set aside.

Mix the eggs, sugar, flour and salt together in a bowl. Add the melted chocolate and butter mixture. Sprinkle in the walnuts. Stir and fold together, being careful not to over-mix.

Spread evenly into the baking pan and bake on the middle shelf of the oven for 15 to 18 minutes, or until a crust has formed but they are still a bit wobbly (you don't want them to be really cooked through like a cake).

Allow the brownies to cool slightly and then cut into squares. Serve with powdered sugar dusted over the top.