Set up your workplace with 2 microwave safe bowls. Stir the dark chocolate and mint chocolate morsels together in one bowl with 3 teas shortening. Microwave in 30 second bursts, stirring in between each burst, for 1 ½ to 2 minutes, until chocolate is melted and smooth.
Quickly repeat with the remaining 1 teas shortening and white chocolate morsels.
Spoon the mint chocolate onto the lined baking sheet, smoothing with an offset spatula.
Working Quickly, drop random spoonfuls of the white chocolate all over the mint chocolate.
Using a butter knife or wooden pick, gently pull the point through the chocolate, creating delicate swirls as desired.