Decorate cookie. Using black FoodWriter marker and ruler, draw lines ¼ in. wide on left side of heart cookie, starting in center and working towards edge.
Using thinned white icing, pipe outline 1/8 in. from edge on right side of heart cookie, starting in center and working around entire right side; flow in. Using thinned black icing, immediately pipe small hearts on right side of cookie.
Using black icing, follow FoodWriter pattern to pipe lines on left side of cookie. Fill in between black lines with white icing.
Let cookies dry 8 to 10 hours.