Place each gelatin mix in a separate bowl; stir 1/2 cup boiling water into each until completely dissolved, about 2 minutes. Stir 1/2 cup cold water into each gelatin mix; refrigerate until completely set, 4 hours to overnight.
Grease a 9-inch cake pan or use a silicone mold. Slice the chilled gelatin into cubes and gently toss together; tip into the prepared cake pan.
Pour 1 cup cold water into a bowl and sprinkle unflavored gelatin over water; let sit until dissolved, 2 to 3 minutes. Add 1 cup boiling water and stir until gelatin is dissolved.
Pour sweetened condensed milk into a blender or food processor. Fill the condensed milk can with double cream and add to the blender with the unflavored gelatin mixture and vanilla extract. Blend mixture until well combined; pour over gelatin cubes in the cake pan. Evenly distribute milk mixture using a spatula.
Refrigerate until set, 4 hours to overnight. Run a knife along the sides of the mold and invert cake onto a plate.
