Preheat oven to 350 degrees. Coat 8x8 pan with cooking spray or parchment paper.
Melt butter and chocolate in a small saucepan over very low heat, continuously stirring. When chocolate is just about melted, remove from heat and continue to stir until mixture is smooth. Transfer mixture to a bowl,stir in sugar, and mix until well blended. Beat in eggs, one at a time. Add water and vanilla and mix until incorporated. Add the flour, baking powder and salt and stir until smooth and no traces of flour remain. Add chips/nuts until blended. Pour into baking pan and bake about 23 minutes, or until set and barely firm in the middle. Cool at least one hour prior to cutting.