Melt 3 tbsp butter in medium glass bowl in microwave; stir in brown sugar until blended. Press into bottom of prepared pan. Place pan in oven 6-8 min, until mixture bubbles.
Melt remaining 1/4 cup butter in same glass bowl. Combine baking mix, corn muffin mix, and granulated sugar in large bowl. Stir in milk, eggs, melted butter, and vanilla.