Preheat the oven to 140°C (280°F) and line baking trays with silicone mats.
Wipe your mixing bowl with vinegar to remove any grease.
Sift together the icing sugar and almond flour - this ensures smooth macaron shells.
Place egg whites and sugar in a bowl over simmering water. Stir gently until the sugar dissolves and the mixture reaches 50°C (120°F).
Whisk the mixture with a stand mixer or hand mixer, starting on low and gradually increasing speed until stiff, glossy peaks form.
Fold in the sifted almond flour and icing sugar using a spatula in a “J” motion. The batter should flow slowly but not be runny.
Transfer to a piping bag fitted with a round tip. Pipe pumpkin-shaped shells onto the silicone mat using a template if needed.
Let the shells rest at room temperature for up to 1 hour until the tops are dry to the touch.
Bake for 15 minutes. Turn off the oven, open the door slightly, and let them rest inside for 30 minutes before removing.
Cool completely before lifting the shells from the mat. Pair shells of similar size.
Make the ganache.
Heat the cream until steaming but not boiling. Pour over the white chocolate chips and stir until smooth.
Chill until set, then whip with a mixer until it reaches stiff peaks.
Transfer to a piping bag fitted with a star nozzle. Pipe ganache onto one shell and sandwich with another.
Use green-coloured melted white chocolate to pipe little leaves for decoration.