1. Coat inside of 8 1/2-by-4 1/2-inch loaf pan with cooking spray. Line with parchment paper by cutting two strips, one the width and the other the length of the pan, overlapping them inside the pan. Both should be long enough to generously hang over edges of pan. Place in freezer to chill for 15 minutes. Beat pistachio ice cream in the bowl of an electric mixer fitted with the paddle attachment until soft but still holding its shape, 1 to 2 minutes. Add pistachios; remove pan from freezer. Transfer pistachio ice cream to pan, forming a 1-inch layer and smoothing with a small offset spatula. Return pan to freezer until the ice cream hardens, about 40 minutes.

2. In a small saucepan, combine cherries, rum, and water. Bring to a boil over medium-high heat (be careful not to ignite); reduce heat; cover pan. Let cherries simmer until all liquid is absorbed, 10 to 12 minutes. Remove from heat; let cool.

3. In a medium bowl, whip cream until soft peaks form; set aside. Soften vanilla ice cream as above. Fold whipped cream into vanilla ice cream. Remove mold from freezer; add a layer of vanilla mixture, smoothing top with a small offset spatula. Return to freezer.

4. When vanilla layer is firm to the touch, about 45 minutes, soften chocolate ice cream as above. Add cooled cherries; mix until just combined. Remove mold from freezer; fill to top with chocolate-cherry mixture. Smooth top with an offset spatula; cover with plastic wrap. Return to freezer until completely hardened, about 4 hours or overnight. To unmold, remove from freezer; dip in very hot water for a few seconds. Sprinkle sliced almonds, if using, over chocolate ice cream to keep ice cream from sliding when turned out. Using the overhanging parchment, gently lift ice cream out of pan. Invert onto cutting board. Use a sharp knife to slice into servings.