Bring the hot water to a boil in a pot.
Pour the boiling water over the gelatin powder and stir vigorously until completely dissolved and the mixture is smooth, about 2 minutes.
Add cold water to the gelatin mixture and stir well to combine.
Pour the liquid gelatin into the brain-shaped mold, filling it evenly.
Place the mold in the refrigerator and let it set for at least 4 hours or until completely firm and wobbling slightly if nudged.
Once set, carefully invert the mold onto a serving plate and gently press to release the gelatin.
Adjust and arrange the gelatin to resemble a gruesome brain, then serve chilled to enjoy its squishy texture and creepy appearance.