Special equipment: a Bundt pan
Pour 2 cups water into a small saucepan and sprinkle the gelatin over the top. Let sit until the gelatin has absorbed liquid, about 1 minute. Bring to a boil over medium heat, stirring often with a wooden spoon to fully dissolve the gelatin. Pour the mixture into a bowl and add the sugar, stirring until dissolved. Let cool until just warm to the touch. Then add the prosecco, club soda and raspberry liqueur; stir until combined. Ladle 2 cups of the liquid into a Bundt pan. Sprinkle with the raspberries and refrigerate until pretty much set, 1 hour.
Once the first layer is set, pour the remaining liquid over the top. Refrigerate for another 3 hours, until fully set.
To unmold, dip the Bundt pan in a small amount of warm water for a few seconds, then cover the pan with a plate and invert so the mold releases onto the plate.
Serve with more raspberries if desired.