Sweetened condensed milk adds an unmistakable richness to the coconut batter for these swirly treats. When marbleizing the coconut and brownie batters, make sure the butter knife reaches the bottom of the pan.
To swirl brownies, work quickly so that batter and filling don't set. Use two large spoons to drop dollops of filling on the first layer of batter about one inch apart. Repeat with next layer of batter and remaining filling. Drag a butter knife back and forth through the layers with the tip touching the bottom of the pan.