Preheat oven to 325°F (160°C) and grease and line an 8-inch pan with parchment paper. In a bowl resting over a pot of barely simmering water, melt chocolate and butter, stirring gently. Remove from heat and stir in sugar and vanilla. Stir in eggs, one at a time. Stir in flour and salt. Fold in white and milk chocolate chips. Bake for 35 to 40 minutes, until a tester inserted in the centre of the brownies comes out clean. Cool before slicing.