Step 1

For the chocolate cake: Prepare and bake the cake according to the package directions for a 9-by-13-inch pan. When completely cool, invert the cake onto a large cutting board. Using a sharp serrated knife, cut the cake into 1-inch cubes. Set aside for assembling.

Step 2

For the chocolate bark: Line a baking sheet with parchment paper. Place the chopped white chocolate and milk chocolate in separate small microwave-safe bowls. Microwave on high in 30-second intervals, stirring well between each, until both are completely smooth, about 1 minute. Dollop both chocolates in the center of the prepared baking sheet, then swirl together with a skewer or small offset spatula (the bark should be about 1/4 inch thick). Sprinkle with crushed peppermint candies. Transfer to the refrigerator to harden, about 15 minutes. Break the bark into large pieces or cut into tree shapes with a cookie cutter. Store in the refrigerator until ready to assemble.

Step 3

For the whipped white chocolate-mocha ganache: Heat the heavy cream and instant espresso powder in a small saucepan over medium heat until just simmering, about 2 minutes. Put the chopped white chocolate in a medium heatproof bowl, then pour the hot cream over. Let sit for about 5 minutes, then stir with a rubber spatula until melted and smooth. Transfer to the refrigerator until cool to the touch but not set, 15 to 20 minutes (it will thicken but not become completely solid). Once cool, whip with an electric mixer on medium-high speed until lightened in texture and color (it will hold soft peaks), about 2 minutes. (Do not overmix. The ganache will become grainy or break if too much air is incorporated.) Transfer to a piping bag fitted with a large star tip.

Step 4

For the milk chocolate-peppermint ganache: Heat the heavy cream in a small saucepan over medium heat until just simmering, about 2 minutes. Put the chopped milk chocolate and peppermint extract in a medium bowl, then pour the hot cream over. Let sit for about 5 minutes, then stir with a rubber spatula until smooth. Set aside at room temperature until thickened and pipeable, about 30 minutes. Transfer to a large piping bag and snip off the end.

Step 5

To assemble the trifle, layer about 1/4 of the cake cubes in the bottom of a trifle dish. Pipe a thin layer of peppermint ganache around the perimeter of the dish, and another squiggle in the center. Pipe small dollops of the white chocolate ganache around the perimeter, then sprinkle all over with about 1/4 of the crushed cookies and 1/4 of the crushed peppermint candies. Repeat until there are 4 layers total, topping the trifle with rosettes of the remaining white chocolate ganache. Sprinkle the remaining cookies and peppermint candy on top, then insert the chocolate bark.

Step 6

Refrigerate for at least 1 hour and up to 4 hours. Use a long-handled serving spoon to scoop.