Line a baking sheet with parchment paper, set aside.
Melt the dark chocolate either in a double boiler or the microwave. If using double boiler, place chocolate in a heat proof bowl or pot over another pot filled with water. Simmer water, and stir chocolate until melted. If using microwave, place chocolate in a microwave safe bowl (I used a glass measuring cup for easy pouring). Microwave in 10-15 second intervals, stirring between each time, until totally melted. Do not over cook, as chocolate will burn easily.
Pour the chocolate onto the prepared parchment paper. Use a spatula to spread it into a rectangle approximately 8 x 12 inches. Allow to cool on the counter until mostly set (but not totally set). About 10 minutes.
Repeat the chocolate melting process with 8 ounces of the white chocolate. Pour over the dark chocolate layer, spreading to cover the dark chocolate.
Repeat the chocolate melting process with the remaining 4 ounces of white chocolate. Transfer melted chocolate to a piping bag with a #8 tip, or a ziplock bag with the corner snipped off. Use the piping bag to make long stripes of chocolate going back and forth across the bark, working from the top to bottom and back up again, until the bark resembles a mummy.
Afix the candy eyeballs around the bark using the wet white chocolate.
Allow bark to set completely in fridge or freezer. While still cold, break into pieces or use a knife to cut into pieces.
To ensure the two kinds of chocolate stay together after cooling, make sure that the first layer of chocolate is partially set, but not totally set, before adding the second layer.