Prepare ice cream: Add the heavy cream and vanilla to a large mixing bowl. Whip with a hand mixer until stiff peaks form. Fold in the condensed milk and whip again until stiff. Fold in ½ cup (20 G) of Fruity Pebbles.
Spread the ice cream across a baking sheet lined with plastic wrap to about a 1-inch (2 ½ cm) thickness. Freeze until frozen solid, at least 4 hours.
Prepare the cereal rounds: Melt the butter in a large pot over medium heat. Add marshmallows and stir until melted and fully incorporated. Add 5 cups (200 G) cereal and mix until combined.
Pour cereal mixture on a greased and lined baking sheet, then press the mixture down to make it more solid. Let cool at least 10 minutes.
With a greased cookie cutter, cut out 16 rounds.
When the ice cream is completely frozen, use the same cookie cutter to cut out 8 rounds of ice cream to fit the cereal rounds.
Sandwich an ice cream round between two cereal rounds.