Spoon half of the cake batter evenly into pan. Spoon or pipe filling into center of cake batter in pan, leaving 1/2-inch border of batter on both inside and outside edges. Using offset spatula or butter knife, gently swirl batter and filling together. Spread remaining batter evenly over top. Place towel on counter, and gently but firmly tap bottom of pan on towel several times to release any pockets of air in batter.