2 sticks sweet, unsalted butter at room temperature
3 squares baking chocolate (each 1 oz), chopped (I don't usually bother to chop)
2 eggs, room temperature
1/4 tsp salt
1 cup sugar
between 1/2 and 3/4 cup flour
1/2 tsp baking soda
1 tsp vanilla
2 Tbsp chocolate liqueur (chocolate-orange, cookies 'n cream....)
6 oz mini semi-sweet chocolate chips
tiny amount of powdered sugar
 

Preheat oven to 350. Line a 9" square baking pan with parchment paper
and butter the inside of the paper, then dust with powdered sugar.
Gently melt baking chocolate and butter in top of double boiler,
over barely simmering water.
Sift flour and baking soda together (I just stir them together).
Beat eggs and salt until lemon-colored. Add sugar slowly, beating
until thick. Add melted chocolate/butter, then vanilla and liqueur.
Stir in flour mixture. Add chocolate chips. Pour into pan and bake
until top is firm but center is soft (22 minutes). Remove from oven,
allow to cool some before dusting powdered sugar on top. Refrigerate
until it solidifies, cut into smaller-than-usual pieces with serrated
knife, and allow to return to room temperature before serving.
(Messy, but more yummy this way.)

The original recipe includes an optional cup of chopped walnuts,
to be added with the chocolate chips at the end, but I never use them.