Combine all of the ingredients in a small bowl and mix together until it begins to form small clusters.
Spread evenly on a baking sheet lined with parchment paper.
Bake at 300°F for about 15 minutes until they are slightly crisp and dried out.
Let cool completely to room temperature. Then store in an airtight container at room temperature for up to a week.
Macarons
Sift together almond flour and powdered sugar into a large bowl and set aside.
In a stand mixer fitted with the whisk attachment, whisk the egg whites on medium speed until they become foamy, then add in the cream of tartar.
Turn the mixer up to medium-high and gradually add in the granulated sugar. Once all the sugar is added, turn the mixer to the highest setting and whisk on high until soft peaks form.
If desired, add in the vanilla bean paste. Then continue to whisk on high until stiff peaks form.
Using a spatula, gently fold in half of the dry ingredient mixture until fully combined. Then gently fold in the remaining dry ingredients.
Continue to gently fold the batter until it reaches a “flowing lava” consistency. You should be able to make a “figure 8” with the batter before it breaks. As soon as you reach this consistency, stop mixing! If you over-mix the batter, the macarons won’t rise properly in the oven.
Scoop batter into a large piping bag fitted with a large round tip. Hold the piping bag perpendicular to the silicone mat and pipe 1 inch rounds.
Pick up the baking sheet and drop it onto the counter 3-4 times to help bring any air bubbles to the surface of the macarons. Then use a toothpick to pop any visible air bubbles to make a smooth surface.
Sprinkle a few rainbow nonpareils on top of the piped macarons before they dry.
Let the macarons sit at room temperature for 30-40 minutes or until a skin forms on the surface. You should be able to lightly touch the surface without any batter being disturbed.
While the macarons are resting, preheat the oven to 300°F.
Bake one sheet at a time on the center rack of the oven for 13-15 minutes or when they do not move on their “feet” when touched.
Let the macarons cool completely on the baking sheet before removing them. They should easily peel off when fully cooled.
Buttercream
In a stand mixer fitted with the paddle attachment, cream together the softened butter and powdered sugar until smooth and creamy.
Mix in the vanilla and beat the buttercream on high for 3-4 minutes until super light and fluffy.
Match up your shells before piping, and then turn one side over. Fill the center with the buttercream, leaving a bit of room around the sides. Then sprinkle the cooled birthday cake crumbs over top of the buttercream, and top with the other shell.
Put the assembled birthday cake macarons in an airtight container, then place them in the fridge to mature for 12-24 hours. Store in the fridge for up to 5 days. However, for the best taste/texture, let them come up to room temperature for about an hour before eating! [Directly out of the fridge they can sometimes be super hard but at room temperature they're perfectly soft and tender!]
Notes
Tip: Clean mixing bowl and whisk attachment of your stand mixer with lemon juice or white vinegar to make sure there are no trace fats on either (if you skip this step, your meringue may not form).