While the shells are cooling, make the filling!
In a medium bowl, add the applesauce, sugar, and salt. Then add in the cornstarch and mix well. In a medium saucepan, add the oat milk and bring it up to a scald. You should see bubbling around the edges. Remove the pan from heat and stir in the applesauce mixture. Add the pan back to the heat, whisking continuously until thickened. Pour into a bowl and cover with plastic wrap. Set aside and allow to cool to room temperature.
In a large bowl or stand mixer bowl add room temperature margarine and whip until light and fluffy (6-8 minutes).
Add the cooled applesauce mixture to the margarine in 3 batches, mixing well with each addition and scraping the sides and bottom of the bowl well. And voilà! You have an apple buttercream!
Scoop the KORU® apple buttercream into a piping bag fit with a ½ inch tip and generously fill a macaron shell and then gently take another shell, placing it on top and pressing together very softly as not to crack it.