Grind up almond flour in a dry food processor and sift through a sieve. Do not force it through the sieve.
Sift confectioners sugar into almond flour with a sieve and a whisk.
Crush cereal in a baggie with a meat tenderizer or rolling pin. Sift crushed cereal into the dry mixture.
Beat egg whites and cream of tartar on medium / high. Slowly add in granulated sugar.
When the soft peak stage is reached add food coloring and vanilla. Mix till stiff peak stage.
Add about ¼ the dry ingredients to the egg whites. Fold with spatula till combined.
Then add the rest of the dry ingredients and gently macaronage (fold) till “lava” texture is reached. The batter should make ribbons that don’t break when you lift the spatula. Try making a figure 8.
Line 2 cookie sheets with parchment, using a tiny dab of batter on each corner as glue.
Add batter to a piping bag using a plain round tip.
Pipe about 1 ½” circles, holding the bag perpendicular to the cookie sheet. To break off batter, make a little swirl motion on top.
Bang sheet on counter to get rid of bubbles and to help tops get smooth.
Let sit for 45 - 60 minutes on the counter uncovered. You should be able to touch the macarons, there will be a skin formed on top.
Preheat the oven to 300 degrees F.
Bake for 10 - 15 minutes. You should be able to see the “feet”.
CEREAL INFUSED MILK
Pour cereal into milk and let it sit for 20 minutes. Strain out cereal. Chill when not using, can make a day ahead.
FROOT LOOPS BUTTERCREAM
In a sauce pan, add sugar, egg yolks, and milk.
Whisk mixture and cook until temperature reaches 160 degrees F. To cook the egg yolks through and dissolve sugar. It should be bubbling and thicker than liquid. Careful not to burn, simmer low for a minute or so.
In a mixer, whip hot strained yolk liquid until light in color and foamy.
When the mixture is cooled to room temp start adding in butter 1 tbsp at a time. Mix until smooth and creamy. It may look worse before it gets better. If it's still too warm or loose, put it in the fridge to chill 10 - 15 mins.
Add in the crushed cereal powder and mix thoroughly.
Separate equal amounts of frosting into small bowls or cups. Color each one a different hue for the rainbow effect you want. I did pink, yellow, blue, and purple gel colors. Go easy on the colors, you can always build it up.
Layout a rectangle of plastic wrap. Smear a line of icing in each color next to each other in rainbow order.
Roll up buttercream like sushi, try to keep the plastic outside. Twist ends and cut off one side.
Drop the rainbow icing log into a prepared icing bag, cut side towards open end, with a preferred tip already on.
Pick out some marshmallows from the cereal and set aside.
Pipe buttercream on one shell and place a marshmallow in the middle. Place top on, repeat.