Red Velvet Chocolate-Swirled Brownie Bars

unsalted butter (1 stick), melted

large egg

light brown sugar, packed

vanilla extract

heaping 1/4 cup unsweetened natural cocoa powder

vials red food coloring, or as needed (I used 2 red Betty Crocker gel food coloring vials)

all-purpose flour

salt, optional and to taste

semi-sweet chocolate chips, melted