Sprinkle gelatin over 1/2 cup cold water in a bowl and set aside to soften. Place bean thread noodles in heatproof bowl and top with 4 cups boiling water or amount needed to cover. Allow noodles to soften for 5 minutes.
Pour 1 jar cocktail sauce into a saucepan and bring to a boil over medium heat. Stir in softened gelatin; cook, stirring frequently, until completely melted. Remove pan from heat.
Combine shrimp, celery, and water chestnuts in a large bowl. Drain noodles and shake off excess water; add to bowl. Add warmed gelatin mixture and stir until well combined.
Lightly grease gelatin mold. Press mixture firmly into mold, making sure all gaps are filled. Cover filled mold with plastic wrap and refrigerate overnight.
Invert mold onto a serving platter (see Cook's Notes). Drizzle cocktail sauce around the edges to create pool of "blood."
This should unmold easily when inverted and smacked down firmly on a flat surface such as a cutting board. If not, try immersing bottom portion of the mold in warm water for a few seconds, then try again. If you want the texture of your brain to be more smooth, like pate, use your food processor instead of a knife when preparing your shrimp mixture. Add the vermicelli AFTER the other ingredients have been pulverized to perfection.
I buy bean thread vermicelli from Asian markets, where they come in a packet of 6 or 8 sections. Use one section of noodles for this recipe.