1Heat oven to 425°F. Spray doughnut pan with cooking spray. In 2-quart saucepan, melt 1/3 cup butter over medium heat. Stir in brown sugar. Heat to boiling, stirring constantly. Stir in 3 tablespoons of the milk; return to boiling. Remove from heat; cool to room temperature.
2Meanwhile, in medium bowl, stir Bisquick mix, granulated sugar, 2/3 cup milk, the vanilla and egg until blended. Stir in melted butter. Spoon batter into resealable food-storage plastic bag; seal bag. Cut off small corner of bag; squeeze bag to pipe batter into pan, using about 1/4 cup for each doughnut.
3Bake 7 to 9 minutes or until toothpick inserted near center comes out clean. Immediately remove doughnuts from pan to cooling rack; cool completely.
4Gradually beat powdered sugar into brown sugar mixture with whisk until smooth, adding remaining 1 tablespoon milk if needed. Glaze doughnuts; sprinkle with pretzels and coconut.