Heat heavy cream in a double boiler until it just starts to simmer. If you don't have a double boiler, just add the cream to a large, heat-safe bowl and place it on top of a large pot with boiling water.
8 ounces heavy whipping cream
Once the cream starts to simmer, remove it from heat and add white chocolate chips. Use a hand mixer or whisk to combine until the chocolate chips are completely melted.
12 ounces white chocolate chips
Add anywhere from 5-10 drops of food coloring and mix to achieve your desired color. Bear in mind that the ganache will lighten slightly in color once it is whipped.
5 drops green food coloring
Place the ganache in the refrigerator for at least 30 minutes to cool completely. Then whip with your hand mixer on high until it aerates and thickens.
Once your cookie cake is cooled and your ganache has thickened, spread a generous amount of ganache onto the Christmas cookie cake. Use a spatula to spread it back and forth, leaving you with a "leafy" texture.
Grab your black writing gel and start at the top and draw swooping, diagonal lines, back and forth from side to side, moving down to the bottom of the tree.
⅔ ounce black writing gel
Place your M&Ms just under the black lines you drew, about 1 mm apart, alternating colors of your choice to resemble hanging Christmas lights.
¼ cup M&M's of choice