Add the egg whites to a mixing bowl and beat until they begin to foam. Add the cream of tartar and sugar and continue to beat until stiff peaks form, about 4-5 minutes.
Add the food coloring and mix lightly until the color is evenly incorporated.
Macaronage
Sift the almond flour and powdered sugar into a medium bowl, discarding any pieces that do not sift through.
Pour the flour and powdered sugar mixture into the egg white mixture. Use a spatula to fold the dry ingredients into the meringue. After about 10-15 folds, begin lightly pressing downward on the batter to remove any excess air. Stop folding when the batter falls from the spatula in a thick ribbon with the consistency of lava. It should be glossy with no visible lumps.
Piping
Add the batter to a piping bag and pipe 1 ½-inch rounds on a baking sheet.
Let the rounds sit for 45-60 minutes. They should appear slightly dry on the outside. Preheat the oven to 300F/150C about 10-15 minutes before the end of the resting period.
Baking
Bake the rounds at 300F/150C for 15-16 minutes. Allow the baked rounds to cool for at least 5-10 minutes before attempting to remove them from the baking sheet.
Pumpkin Buttercream
Beat the butter until light and fluffy (about 3 minutes). Add the powdered sugar, pumpkin puree, and salt and mix until smooth.
Final Assembly
Transfer the buttercream to a piping bag and pipe about 2 tsp between each pair of macaron shells.
Chill the macarons for at least two hours to allow the buttercream to solidify between the layers before serving.