Line a 9 x 12 pan with foil. Preheat oven to 350.

Melt the butter and chocolate in a medium saucepan over low heat on the stove top, stirring constantly until smooth.

Remove from heat and stir in the sugar, then the eggs, one at a time.

Sift together the cocoa powder and cornstarch in a small bowl. Stir this mixture into the chocolate batter. Beat the batter briskly, until smooth.

Add nuts to batter (optional). Pour batter into the lined pan.

Bake for 20 mins, or until the brownies are just set in the center. Do not over bake. Cool completely before slicing.