Sift almond flour and 1 cup powdered sugar together twice into a bowl, discarding any large pieces that don't pass through. Set aside.

In a spotlessly clean metal bowl, whip egg whites with cream of tartar on medium speed until foamy. Gradually add granulated sugar one tablespoon at a time, then increase to high speed and whip until stiff, glossy peaks form, about 4-5 minutes.

Add a drop of yellow food coloring if using, then fold in the almond flour mixture in three additions using a flexible spatula. Use a J-motion to fold, pressing batter against the side of the bowl. Test consistency by letting batter fall—it should flow like lava and a ribbon should disappear within 10-15 seconds.

Transfer batter to a piping bag fitted with a round tip. Pipe 1.5-inch circles onto parchment-lined baking sheets, spacing them 2 inches apart. Tap the sheet firmly on the counter 5-6 times to release air bubbles.

Let shells rest uncovered at room temperature for 30-60 minutes until a skin forms and they feel dry to the touch. Preheat oven to 300°F during the last 15 minutes of resting.

Bake one sheet at a time for 14-16 minutes, rotating halfway through. Shells are done when they don't wobble when gently touched and peel cleanly from parchment. Cool completely on the baking sheet.

For buttercream, beat softened butter on high speed for 3 minutes until fluffy. Add sifted powdered sugar and beat for 2 minutes. Add strained passion fruit pulp and heavy cream, beating until smooth and glossy.

Match shells by size. Pipe or spread buttercream on flat side of one shell, then sandwich with a matching shell, pressing gently to spread filling to the edges. Refrigerate assembled macarons in an airtight container for 24 hours before serving for best texture.