Pour 3 cups whipping cream in the bowl of a stand mixer, then add 1/2 cup confectioners' sugar. Whip on high speed until stiff peaks form, about 2 minutes. Scrape the whipped cream out of the bowl and set aside for cake assembly.
Pour the remaining 3 cups whipping cream into the stand mixer bowl, then add the cocoa powder, vanilla extract and remaining 1/2 cup confectioners' sugar. Whip on high speed until stiff peaks form, about 2 minutes. Set this chocolate whipped cream aside for cake assembly.
Assemble the cake on a round serving platter or cake stand. Place 2 tablespoons of the plain whipped cream in the center of the platter. Spread it into a very thin layer, forming a 10-inch round to secure the first layer of wafers. Place 9 wafers, side by side, around the inner edge of whipped cream. Fill the center of the wafer circle with 3 more wafers (break 1 of the 3 wafers in half to fill the center neatly like a puzzle).
Spread a third of the chocolate whipped cream (about 1 cup) in an even layer on top of the wafers using a small offset spatula. Be sure to push the whipped cream almost to the edge of the wafers, leaving just a bit of the wafers exposed. Repeating the same pattern, make another wafer layer with 12 wafers. Next, place a third of the whipped cream (about 1 cup) on top of the wafers in an even layer. Top the whipped cream with another layer of wafers. Repeat this process to create 4 more layers, alternating between the plain whipped cream and chocolate whipped cream, each separated by a layer of 12 wafers. Place the cake into the refrigerator to set up 8 hours or up to overnight.
For the cherry sauce: Meanwhile, combine the frozen cherries, granulated sugar, honey, salt, lemon juice and 1/2 cup tart cherry juice in a medium saucepot. Cook over medium-high heat until the sugar dissolves and the cherries soften, about 8 minutes. Whisk the cornstarch with the remaining 1 to 2 tablespoons of tart cherry juice in a small mixing bowl to create a slurry. Drizzle the slurry into the cherry mixture, stirring constantly, then cook at a low simmer, stirring occasionally, until the sauce develops a smooth velvety texture, 2 to 3 minutes. Put the sauce in the refrigerator to cool until the icebox cake is ready to serve.
Remove the cake from the refrigerator. Top with the cold cherry sauce and serve immediately.