Special equipment: a 4-quart trifle bowl or a 4-quart glass bowl
For the trifle: Combine the chocolate and butter in a medium bowl. Put the bowl over a barely simmering saucepan of water and stir occasionally until the chocolate has melted and the mixture is smooth, about 5 minutes. Set aside.
In another medium bowl, using a fork, gently mash the chestnut puree into 1/2-inch chunks.
In a stand mixer, fitted with a whisk attachment, beat the cream until thick. With the machine running, gradually add the sugar and continue to beat until the cream holds stiff peaks. Add half of the whipped cream to the chestnut puree and, using a spatula, fold it in until incorporated. Set aside.
Stir the orange zest and 1/3 of the remaining whipped cream into the chocolate mixture until smooth, then fold in the remaining whipped cream. Set aside.
To assemble the trifle: In the bottom of a 4-quart trifle bowl, arrange a single layer of pound cake slices, trimming the cake slices, if necessary, to fit. Drizzle the pound cake with about 1/4 cup of the orange liqueur or orange syrup. Spread 1/2 of the chestnut cream on top. Arrange another layer of pound cake slices on top and drizzle with 1/4 cup orange liqueur. Spread 1/2 of the chocolate cream on top. Repeat the layers using the remaining cream and liqueur, finishing with a chocolate cream layer.
For the garnish: Using a vegetable peeler, shave the chocolate bar on top of the trifle and sprinkle with orange zest. Cover the trifle with plastic wrap and refrigerate for at least 3 hours or overnight. Allow the trifle to stand at room temperature for at least 1 hour before serving.
In a small saucepan, bring the orange juice and sugar to a boil over medium-high heat. Simmer, stirring occasionally, until the sugar has dissolved, about 5 minutes. Cool for 20 minutes.
Chestnut puree can also be made by draining 1 (14-ounce) can of whole chestnuts in water and placing in the bowl of a food processor. Add 1/4 cup sugar, 1/8 teaspoon fine sea salt, and 2 tablespoons water. Blend until smooth, adding more water, 1 tablespoon at a time until the mixture is a spreadable consistency.