Heat oven to 350°. Line the bottom of a 9x13” baking pan with parchment paper and grease entire pan with butter and flour.
Melt butter and white chocolate chips in a medium saucepan over low heat, stirring constantly, just until melted. (Mixture may appear curdled.) Remove from heat and cool.
Stir remaining brownie ingredients together in a large bowl. Stir in chocolate mixture. Spread in pan and bake about 30 minutes, until a toothpick comes out clean. Cool completely.
Make the glaze: Whisk all ingredients until smooth and spreadable. Spread over cooled brownies and cut into bars.