Preheat the oven to 350F/177C. You can prepare the layers in 3, 8-inch rounds or use a 13x18-inch large sheet pan.
In a large mixing bowl, cream together the softened butter and sugar. Then add in the eggs and vanilla extract. Mix again until creamy. Prepare the coffee: dissolve the 2 tablespoons of granules in 1/4 cup water, then add to the batter along with the milk. In a separate bowl, combine all the dry ingredients: flour, cocoa powder, salt and baking powder. Mix together, then sift into the cake batter to get rid of any clumps. Using a hand whisk, mix the flour into the batter just until combined. Be careful not to over-mix the batter! Gently fold in melted chocolate at the very end.
Divide the cake batter between the prepared baking pans. Bake the layers in preheated oven for 30 minutes, or until a toothpick inserted into the center comes out clean. Once the cake has cooled completely, cut the cake into cubes. The cake can be prepared ahead of time; just make sure to wrap the layers in plastic so they don't dry out!
For Peppermint Syrup:
Prepare the peppermint syrup. Combine the sugar with boiling water and stir until completely dissolved. Add a few drops of peppermint oil. Oil is much stronger than extract so increase the amount if using an extract. Transfer the syrup into a dispenser bottler and set aside (can also spoon the syrup over the layers).
For White Chocolate Mousse:
Prepare the white chocolate mousse. Place white chocolate chips into a large bowl and melt in the microwave until smooth; can also use a double boiler. Add the sweetened condensed milk and more peppermint oil. Stir immediately until the mixture is smooth. If clumps form, use a mixer to whisk the mixture until smooth.
In a separate bowl, whisk cold heavy cream until stiff peaks form. Add a few scoops of whipped cream to the white chocolate base and fold gently. Add the chocolate base gradually to the whipped cream, folding gently. Once all combined, divide the mousse between two bowls. Keep one bowl white and color the other one red.
Assemble the dessert in a large trifle bowl or use small, individual cups. Layer the chocolate cake first, soak with the peppermint syrup, add white chocolate mousse, add more chocolate cake, peppermint syrup, red mousse, top with more chocolate cake pieces, and garnish with crushed candy cane pieces.
Keep dessert refrigerated for 4 to 6 hours, or make it a few days ahead of time.