When lining the pan, press the foil neatly into the corners, and smooth the bottom and sides. If necessary, brush sides of pan with butter to help foil stay in place.

Ingredients
1/2  cup (1 stick) unsalted butter, plus more for foil    
8  ounces semisweet chocolate, chopped    
1 1/4  cups sugar    
3/4  teaspoon salt    
3  large eggs    
3/4  cup all-purpose flour (spooned and leveled)    
1  cup chopped walnuts or pecans (optional)

Directions 
1.   Preheat oven to 350F. Line bottom and sides of an 8-inch square baking pan with foil; butter foil. Set aside. 

2.   Place butter and chocolate in a medium heatproof bowl set over (not in) a saucepan of simmering water; stir frequently, until almost melted. Remove from heat; stir until completely melted. 

3.   Whisk in sugar and salt until smooth; whisk in eggs. Gently whisk in flour just until smooth (do not overmix). Fold in nuts, if using. 

4.   Spread batter evenly in prepared pan. Bake until a toothpick inserted in center comes out with a few moist crumbs attached (they should form a ball when rolled between your fingers), 45 to 50 minutes. Cool completely in pan. 

5.   Use foil to lift from pan; peel off and discard. Cut into 16 squares.
 
Note: Instead of dipping the dry measuring cup into the flour, spoon flour into the cup; then level flour with the straight edge of a knife.     

Source:  www.marthastewart.com