Step 1: Sift almond flour and powdered sugar together twice to remove lumps.
Step 2: In a clean bowl, whip egg whites with cream of tartar and salt until foamy.
Step 3: Gradually add granulated sugar while whipping to stiff peaks.
Step 4: Fold in sifted dry ingredients and 2 tsp espresso powder gently until batter flows like lava.
Step 5: Pipe 1.5-inch circles on parchment-lined baking sheets, then tap to release air bubbles.
Step 6: Let shells rest 30-60 minutes until dry to the touch.
Step 7: Bake at 300°F (150°C) for 15-18 minutes, rotating halfway.
Step 8: Cool completely on wire racks before removing from parchment.
Step 9: For buttercream: Beat butter until light, then add powdered sugar, espresso powder, cream and vanilla.
Step 10: Pipe buttercream onto half the shells and sandwich with remaining shells.
Step 11: Refrigerate 24 hours for best texture before serving.