Place a rack just below center of oven and preheat to 300 degrees.
Line an 8-inch-square pan with foil and brush with melted butter.
In a bowl, whisk flour and salt together.
In top of a double boiler set over barely simmering water, or on low power in a microwave, melt remaining butter and chocolate together. Stir often and remove from heat when a few lumps remain. Stir until smooth.
In a mixer, beat eggs and sugar together until thick and pale yellow. Add chocolate mixture and vanilla and mix at low speed until smooth. Add dry ingredients and mix 30 seconds, then finish mixing by hand, adding nuts if using.
Pour into prepared pan and bake 50 to 60 minutes, until top is dry.
Let cool in pan, then lift out and cut into bars or wrap in foil.