Preparation

Prepare the cherries

• If using frozen cherries, put them in a strainer over a bowl for 4 to 8 hours until they are completely thawed and have released at least 3 Tbs. of juice; reserve the juice. Halve and pit fresh or frozen cherries. Reserve several for garnish, if you like.
• In a large bowl, toss the cherries with 3 Tbs. of the kirsch, 2 Tbs. of the sugar, the lemon juice, and salt. Stir occasionally, until the sugar dissolves, about 5 minutes.
• Heat a 12-inch heavy-duty skillet over medium-high heat until a cherry sizzles when you toss it in, about 2 minutes. Scrape in the cherry mixture and cook over medium heat, frequently scraping the bottom of the pan with a wooden spatula to prevent burning, until the syrup has been reduced to a thick, sticky glaze, about 2 minutes.
• Scrape the cherries into a bowl. If using frozen cherries, pour the reserved juice into the hot skillet and simmer until the juice is thickened and there are large, foamy bubbles, 1 to 2 minutes. Scrape into the bowl of cherries. Season to taste with sugar and lemon juice. Refrigerate until cool, about 30 minutes.

Start assembling the cake

• Cut the cake horizontally into two even layers. Turn the top layer onto a cutting board, cut side up. Put the bottom layer, cut side up, in a clean 9×3-inch round springform pan.
• Stir the remaining 2 Tbs. kirsch, 1/4 cup sugar, and 1/4 cup cool water in a 1-quart saucepan over medium heat until the sugar dissolves, about 3 minutes.
• Brush all of the clear syrup evenly on the cut sides of both cake layers to moisten. Then, brush the cut sides of both cake layers with most of the juice from the cherry mixture.
• Pour all of the cherries and remaining juice on the cake layer in the pan, and arrange in a single layer.

Make the mousse

• Place 6 oz. of the white chocolate and 1/4 cup water in a medium stainless steel bowl. In a 12-inch skillet, bring 1 inch of water to a simmer. Remove the skillet from the heat and wait 30 seconds. Set the bowl of white chocolate in the water. Let stand for 1 minute to soften the chocolate, then stir until the chocolate is melted and smooth. (If the chocolate doesn’t melt, return the skillet to the heat briefly.)
• 删除the bowl from the skillet and let the chocolate cool to 85°F, about 4 minutes. If the chocolate cools too much, return the bowl to the warm water for a few seconds before the next step.
• Whisk 1 cup of the cream in another medium bowl just until thickened and barely beginning to hold a shape—it should be fluffy but still pourable. Scrape the cream into the bowl of chocolate and fold carefully but quickly just until the two are combined. It will seem very soft, but it will firm up later.
• Immediately pour the mousse on top of the cherries, and use an offset spatula to level it. Set the top half of the cake on top. Press it gently to level. Cover the pan and refrigerate for 4 to 12 hours to firm the mousse.

Make the white chocolate shards

• In a small stainless steel bowl set in a skillet of very hot water, melt the remaining 6 oz. of white chocolate.
• Pour the chocolate onto a 16-inch-long piece of waxed paper, and use an offset spatula to spread it into a thin layer to the edges of the paper. Cover the chocolate with a second sheet of waxed paper and smooth it over the chocolate.
• Roll the paper and chocolate into a 1-inch tube. Refrigerate it until the chocolate has hardened completely, at least 2 hours.

Finish the cake

• Run a thin knife dipped in hot water and dried around the cake, and then release the sides of the springform pan and remove. Run a large metal offset spatula (or cake lifter) under the cake and slide it onto a cake plate.
• Whip the remaining 1 cup cream in a chilled medium bowl to firm peaks.
• 删除the white chocolate tube from the fridge and unroll it quickly (while the chocolate is still cold) to crack it into long, curved shards. Slide a thin metal spatula under the chocolate to release it from the bottom sheet of waxed paper.
• Top the cake with the whipped cream, leaving a 1/2-inch border, reserved cherries, if using, and white chocolate shards (the warmth of your fingers will soften the shards, so handle them with a spatula or tongs). Slice and serve.

Make Ahead Tips

The cake can be assembled up to 12 hours ahead and refrigerated. Whip the cream and garnish the cake just before serving.