In a food processor, add 3/4 cup (175 mL) of icing sugar and walnut crumbs and pulse. Set aside.

With a stand mixer on medium speed, whisk egg whites until foamy. Reduce speed to low while adding remaining icing sugar (1/4 cup [50 mL]), then increase the speed to high, and whisk until stiff peaks form (approximately 8 minutes).

Sift walnut mixture over egg whites, and gently fold until the mixture is smooth and shiny. If desired, for a variation of colours, divide macarons batter into three bowls and fold in 1 to 2 drops of food coloring into each bowl before piping and baking.

Transfer the mixture to a pastry bag fitted with a 1/2-inch (1 cm) plain round tip, and pipe 3/4-inch (2 cm) rounds 1-inch (2.5 cm) apart on a parchment lined baking sheet. To flatten any peaks, dip your finger in water and gently press down on the macarons.

Tap the bottom of each baking sheet to release any trapped air bubbles in the macarons. Let stand at room temperature for 15 minutes before baking.

Place 1 baking sheet in the oven at a time and turn oven down to 325 F. Bake for 12-15 minutes. After each batch, increase oven temperature to 375 F, for 5 minutes then reduce back to 325 F for the next batch. Continue until all macarons are baked. Let cool before removing from parchment.

Limoncello Buttercream:
For the buttercream, combine butter, sugar, limoncello and lemon zest in a stand mixer and whip on medium-high speed until light and fluffy, about 10 minutes.

Sandwich two similar sized macarons with 1 tsp (5 mL) of filling.

Serve immediately or stack the macarons between layers of parchment in an airtight container and freeze for up to three months.