Dissolve gelatin in 3 Tbsp. cool water in a small bowl. Let stand until bloomed, about 2 minutes.
Meanwhile, combine hot coffee and sugar in a loaf pan or large resealable container. Gently stir gelatin mixture into coffee until combined (do not stir too vigorously or air bubbles will appear). Chill until firm, about 2 hours.
Using an electric mixer on medium-high speed or a whisk, whip cream in a large bowl until soft peaks form. Add sugar and vanilla and whip to medium peaks.
Blend coffee, syrup, cream, sugar, vanilla, and 3 cups ice in a blender until smooth.
Cut coffee jelly into 3/4” cubes. Fill 4 tall glasses with 1/2 cup cubed coffee jelly each. Spoon 3 Tbsp. whipped cream over each and spread to create a solid layer of cream. Divide frozen iced coffee (about 1 cup each) among glasses on top of cream layer. Top with more whipped cream and serve immediately with bubble tea straws.
