1Heat oven to 350°F (325°F for dark or nonstick pans). In medium bowl, mix crust ingredients. Press in ungreased 13x9-inch pan. Bake 8 minutes; cool 10 minutes.
2In medium bowl, stir brownie ingredients until blended. Carefully spread batter over cooled crust. Bake 24 to 26 minutes or until toothpick inserted 2 inches from side of pan comes out clean or almost clean. Cool completely on cooling rack, about 1 hour.
3In medium bowl, beat powdered sugar, 2 tablespoons butter, melted chocolate and the vanilla with electric mixer on low speed until combined. Beat in 1 tablespoon milk until blended. Beat in additional milk, 1 tablespoon at a time, until frosting is desired spreading consistency. Frost cooled brownies; sprinkle with crushed pretzels. Cut into 8 rows by 4 rows.