Line baking trays with parchment or silicone mats. Let egg whites come to room temperature. Sift almond flour and powdered sugar together.

Whip egg whites in a clean bowl until foamy. Add cream of tartar, then slowly add granulated sugar. Beat until stiff, glossy peaks form. Add vanilla.

Fold in sifted almond flour and powdered sugar in thirds. Mix gently until the batter flows like lava and forms a figure eight.

Pipe the batter onto trays using a piping bag with a round tip. Tap trays to release air bubbles. Use a toothpick for remaining bubbles.

Let piped shells rest uncovered until dry to the touch (about 30–40 minutes).

Bake at 150°C (300°F) for 12–15 minutes, rotating trays halfway. Shells should have feet and not move when touched. Cool completely.

Heat sugar and water to 240°F.

Beat egg yolks until pale. Slowly pour in hot syrup while mixing. Beat until cool, then add butter piece by piece. Finish with vanilla and salt.

Match shells by size. Pipe buttercream onto one side and sandwich with the matching half.

Store in an airtight container and refrigerate for at least 24 hours before serving. Let come to room temperature before eating.