In a 2- to 3-quart pan over low heat, frequently stir butter and semisweet chocolate chips until melted and smooth. Remove from heat and stir in sugar, eggs, 2 tablespoons water, vanilla, flour, and baking soda until well blended. Cool about 10 minutes, then stir in white chocolate chips.
Spread batter evenly in a buttered and floured 8-inch square baking pan.
Bake in a 350° oven until edges feel firm to touch and begin to turn a shade darker, 25 to 30 minutes.
Run a knife between pan rim and brownie. Let cool in pan on a rack, about 1 hour. Cut into 9 or 16 squares.
Note: Nutritional analysis is per small brownie.