Prepare a piping bag fitted with a large round tip or with the tip of the bag cut off. Place the bag in a tall drinking glass so it will be easier to fill. Set aside.
In a small bowl, sift together the oat flour and powdered sugar. Set aside.
Add the egg whites to the bowl of a stand mixer fitted with the whisk attachment. Beat the egg whites on medium-high speed. Once the egg white are foamy, add the cream of tartar. Then, slowly add the granulated sugar, one spoonful at a time.
Add the vanilla and food coloring, if using, and continue to beat until the meringue has formed stiff peaks.
Remove the bowl from the mixer and carefully fold ⅓ of the flour mixture into the meringue. Then add the rest of the flour mixture. Continue folding and stirring until just before the figure 8 stage.
Transfer the batter to the piping bag. Pipe 1-inch circles of batter onto the macaron mat or parchment with 1-inch circles drawn on the under side of the paper. Slam the tray down hard on the counter to cause air bubbles to rise to the surface. Use a toothpick to pop any large bubbles and smooth batter over the hole. Repeat with the second tray.
Allow the batter to sit for 40 minutes to dry at room temperature before baking. Meanwhile, preheat the oven to 300°F.
Bake the Macaron Shells:
Bake the macarons shells, one tray at a time, for 12-15 minutes, rotating once after 7 minutes.
Remove the shells from the oven and allow them to cool completely on the mat before removing. Meanwhile, make the buttercream.
Make the Buttercream Filling:
In standing mixer fitted with the whisk attachment, beat butter at medium-high speed until smooth, about 20 seconds. NOTE: If using a hand-held mixer, increase mixing times by at least 50%.
Add powdered sugar and salt, and beat at medium-low speed until most of the sugar is moistened, about 45 seconds.
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.
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 buttercream to a piping bag fitted with a round or decorative tip. Pipe the buttercream 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.
Decorate the Macarons (optional):
For luster dust, add a small amount of dust to a painters palette. Use a disposable transfer pipette to add a few drops of Everclear or vodka to the dust. Stir the dust with a food grade paintbrush to make a liquid suspension of the dust. Carefully paint the dust on the outside tops of the macaron shells. Prepare more dust suspension as needed.
To add a heart sprinkle on top, carefully add a tiny amount of buttercream on one side of the sprinkle and place it in the middle of the top of a macaron sandwich cookie. Repeat as desired.
Storage:
Store the assembled cookies in an airtight container in the refrigerator for up to several days.
If refrigerated, allow the cookies to come to near room temperature before serving.
Notes
100 grams of egg whites from large eggs is between 3 and 4 eggs. I highly recommend weighing it out.5 ounces of powdered sugar is about 1¼ cups.Use the buttercream immediately or store in refrigerator. If refrigerated, allow frosting to come to room temperature before use. Buttercream can also be frozen. You may freeze the macaron shells or assembled macarons. Thaw before serving. Nutritional information is only an estimate and will vary based on your ingredients and substitutions. For this recipe, the filling is not included in the calculation.You're Gonna Bake It After Allbakeitafterall.com