Line a baking tray with waxed paper or a silicone mat. Prepare a piping bag with a round nozzle (Wilton 2A).

Sift the oat flour with the icing sugar and set aside.

Beat the egg whites on medium speed until foamy. Add the cream of tartar and continue beating until waves form in the foam.

Add the sugar gradually while continuing to beat.

Increase the speed to high and beat until stiff peaks form. At this stage, add the food coloring if desired.

Gently fold in the oat mixture with enveloping movements until the dough falls from the spatula in a ribbon that breaks before forming a figure 8.

Place the mixture in a piping bag and pipe 3 cm circles onto the baking tray, leaving space between each one.

Tap the tray against the table to remove air bubbles. Use a toothpick to pop any large bubbles.

Let rest for 30 to 35 minutes at room temperature, until a light crust forms on the surface.

Preheat the oven to 150 °C (300 °F) and bake for 15 minutes.

Allow to cool completely before removing.

Pair shells of similar size.

Using a piping bag, place filling on the flat side of one shell.

Cover with another shell to form a sandwich.