• Place a macaron mat on each of 2 baking sheets. You can also use parchment paper with 1-inch circles drawn on underneath. Set these aside.
• Place a large round tip, like a #12, in a disposable piping bag. Fold over the top of the bag and place it in a tall glass or in a piping bag holder so it will be easy to fill with the batter. Set this aside.

Prepare the Batter:

• Sift together the oat flour and the powdered sugar in a medium-sized bowl, then whisk to combine them completely. Set this bowl aside.
  126 grams oat flour, 126 grams powdered sugar
• Place the egg whites in the bowl of a stand mixer fitted with the whisk attachment and beat on medium-high until frothy. Add the cream of tartar, then slowly add the granulated sugar, one spoonful at a time, with the mixer still running.
  100 grams egg whites, ¼ teaspoon cream of tartar, 90 grams granulated sugar
• Add the vanilla extract and the food coloring gel. Continue to beat the mixture until the meringue forms stiff peaks. Remove the bowl from the mixer.
  1 teaspoon vanilla extract, yellow and green food coloring gel
• Using a rubber spatula, carefully fold ⅓ of the flour/powdered sugar mixture into the meringue. Then add the remaining ⅔ of the flour mixture and continue folding until the ingredients are fully incorporated and the batter falls off the spatula in clumps, just before you reach the figure 8 stage. See note.
• Once the batter is the proper consistency, transfer it to the prepared piping bag. Pipe 1-inch circles of batter onto the first macaron mat.
• Once the batter has been piped on a full tray, slam the tray down hard on the counter or a table several times to force any air bubbles to the surface. If desired, take a toothpick and pop large bubbles then smooth out the hole left after the bubble pops.
• Pipe the batter onto the second macaron mat. You will not fill the entire mat. Add a heart sprinkle to the top of each shell. You can leave half of them without a sprinkle to be the bottom of the cookie, if desired.
• Allow the batter to dry at room temperature for 40 minutes. Meanwhile, preheat the oven to 300°F.

Bake the Macaron Shells:

• Bake the macaron shells for 12-15 minutes, rotating the pan once about 7 minutes into the bake. Only bake one pan at at time.
• Allow the shells to cool completely on the mats before removing them. To remove more easily, push up on the mat from underneath to help them pop off. If you have trouble with them sticking, that may mean they are underbaked. Some of the shells in the center of the tray may have a little bit of sticking. Very carefully use a stiff metal spatula to pry them off the mat, taking care not to rip the shell.
• The cooled shells can be stored in a single layer in an airtight container in the refrigerator or freezer, or you can fill them immediately. If you must stack them, separate them with layers of parchment paper so they don't stick together.

Make the Filling:

• In a stand mixer fitted with the whisk attachment, beat butter and salt at medium-high speed until smooth, about 20 seconds. NOTE: If using a hand-held mixer, increase mixing times by at least 50%.
  10 tablespoons salted butter, ⅛ teaspoon salt
• Add the powdered sugar and beat at medium-low speed until most of the sugar is moistened, about 45 seconds.
  5 ounces powdered sugar
• Scrape down bowl and beat at medium speed until mixture is fully combined, about 15 seconds; scrape bowl, add vanilla extract and heavy cream, and beat at medium speed until incorporated, about 10 seconds.
  1½ teaspoons vanilla extract, 1 tablespoon heavy cream
• Increase speed to medium-high and beat until light and fluffy, about 4 minutes, scraping down bowl once or twice. This step is important to create a smooth and whipped buttercream.

Assemble the Macarons:

• Because the macarons will be sandwiched, pair up the shells by size. You want the top and bottom of the sandwich cookie to match as closely in size as possible.
• Add the filling to a piping bag fitted with a medium-sized round tip, such as #12. Pipe the filling on the flat side of one shell and carefully place the flat side of the other shell on top. Repeat with remaining pairs until all have been sandwiched.
• Roll the sides of each macaron in crushed peppermint so it sticks to the buttercream.