Mix the cream and thandai syrup ( or powder) well and let stand for atleast half hour.
Sprinkle the gelatin over 2 tablespoons water in a small bowl. Let stand for 5 minutes ( blooming the gelatin)
Heat the cream in a thick bottomed saucepan (while stirring often) until it just bubbles around the edges.
Take off heat and stir in the bloomed gelatin.
Heat again for few minutes while stirring. This is to dissolve the gelatin. Do not let the mixture boil though.
Take off heat and strain.
Pour into the chilled tart case. Sprinkle dried rose petals on top if using.
Refrigerate for atleast 3 hours and upto overnight.
Remove from tart pan and serve cold !