1Heat oven to 400°F. Place paper baking cup in each of 12 regular-size muffin cups.
2In 10-inch skillet, melt butter over medium-high heat. Add rhubarb; cook about 5 minutes or until softened. Stir in sugar and vanilla. Cook 3 to 5 minutes longer. Remove from heat. Stir in raspberries. Set aside.
3In large bowl, stir together muffin mix, milk, melted butter and eggs. Fold in rhubarb mixture. Spoon batter evenly into muffin cups. Top each with Streusel Topping from muffin mix box.
4Bake 18 to 21 minutes or until center muffin springs back when touched lightly in center. Remove from muffin cups. Cool slightly before serving.