Prepare a piping bag fitted with a large round tip or with the tip of the bag cut off. Set aside.
Sift together the oat flour and powdered sugar in a small bowl. Set aside.
In 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 and pipe 1-inch circles onto the macaron mat or parchment with 1-inch circles drawn on the under side of the paper.
Slam the tray on the counter several times to cause air bubbles to rise to the surface. Use a toothpick to pop any large bubbles and smooth batter over the hole.
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 for 12-15 minutes, rotating once after 7 minutes.
Remove the cookies from the oven and allow them to cool completely on the mat before removing.
Make the Filling:
Sprinkle the apples with lemon juice.
Combine dry ingredients in large bowl.
Add the apples to the dry ingredients and toss to mix. Add the vanilla and cream and mix thoroughly. Set aside.
Melt the butter in a skillet.
Add the apple mixture and cook approximately 8-10 minutes, until the apples are soft and the liquid has turned golden and thickened.
OPTIONAL: If the apple mixture appears thin, combine the cornstarch and water in a small bowl. Add to the apple mixture, stir and continue to cook for a few minutes until thickened.
½ teaspoon cornstarch, 2 teaspoons water
Cool the filling, either at room temperature or in the refrigerator, and use immediately or store in the fridge for up to a week.
Make the Buttercream:
In a stand mixer fitted with whisk attachment, beat butter at medium high speed until smooth, about 20 seconds.
Add powdered sugar and salt and beat at medium-low speed until most of the sugar is moistened, about 45 seconds.
Scrape down the bowl and beat at medium speed until mixture is fully combined, about 15 seconds. Scrape bowl.
In a small bowl, dissolve espresso powder in warmed heavy cream and vanilla. Add to mixer bowl and beat at medium speed until incorporated, 10 seconds.
Increase speed to medium-high and beat until light and fluffy, about 4 minutes, scraping down bowl once or twice.
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 as possible.
Add the buttercream to a piping bag fitted with a round or decorative piping tip.
Pipe a buttercream border on the flat side of each bottom shell.
Carefully spoon the cooled apple mixture into the center of the buttercream dam.
Holding the top shell by the edges, gently place the flat side on top of the filling, pressing down slightly to adhere. Repeat with the remaining pairs. You may have leftover apple filling.
Store the assembled cookies in an airtight container at room temperature or in the refrigerator for several days. If refrigerated, allow the cookies to come to room temperature before serving.
Notes
I highly recommend weighing the ingredients listed in grams with a digital kitchen scale. French macarons are notoriously finicky and require precise measurements.Use cracked whole eggs, not store bought egg whites. 100 grams of egg whites (to make both green and pink shells) is the equivalent of the whites from about 3⅓ large eggs."Macaronage," the process of folding the dry ingredients into the meringue, is the trickiest step of macaron making. The batter will be too thick if you under mix it or too thin if you over mix it. I refer you to two resources with visuals as to when the batter is ready to be piped: America's Test Kitchen and Le Cordon Bleu.5 ounces of powdered sugar is about 1¼ cups.Store the cookies in an airtight container in the refrigerator for a few days to allow them to mature and improve in texture. If refrigerated, allow the cookies to come to near room temperature before serving so the buttercream can soften. Unfilled shells or assembled macarons can be frozen.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