Prepare Jello gelatin according to directions except only use 1 cup of cold water instead of two. Let firm up for at least 4 hours or you can make the day ahead. Cut into about 1-inch squares. Mix reserved juice from the can of tart cherries with maraschino cherry juice. Prepare instant Jello pudding per the instructions using 2-1⁄2 cups milk instead of three. Let pudding set up for 5 minutes. Cut the cake or brownies into about 1-inch square pieces; you do not have to be very precise. Layer half of the cake into the bottom of a trifle bowl or clear glass serving bowl. Spoon some of the cherry juice mixture onto the cake, enough to moisten the cake but not drown it. Layer 1 can of drained tart cherries over the moistened cake, making sure the cherries go all the way out to the edges so they show up in the layers. Top with half of the pudding. Top with the cubed Jello, then add the remaining pudding, another layer of cake (moisten the same way as the first layer), and the remaining can of drained cherries. Whip the cream on high speed with the sugar until soft peaks form. Spread over the top of the trifle. Top with a dusting of cocoa powder, some grated chocolate or chocolate curls, and maraschino cherries. Refrigerate for about an hour to let the flavors meld or up to several hours ahead.
To make chocolate curls, take a bar of chocolate and warm it for about 15 seconds in the microwave. Run a vegetable peeler down the side of the chocolate. If curls do not form then the chocolate is not warm enough; warm a few more seconds. Keep cold until using.