Preheat oven to 350°. Place butter in a 13x9-in. baking pan; heat pan in oven 3-5 minutes or until butter is melted. Meanwhile, in a large bowl, combine flour, 1-1/4 cups sugar, milk and vanilla until well blended. In a small bowl, mix cocoa and remaining sugar., Remove baking pan from oven; add batter. Sprinkle with strawberries and cocoa mixture; pour boiling water evenly over top (do not stir). Bake 35-40 minutes or until a toothpick inserted into cake portion comes out clean. Let stand 10 minutes. Serve warm with whipped cream and additional strawberries.