Preheat the oven to 200°F. Combine the icing sugar and almond flour in a medium bowl. If grinding your own nuts, combine nuts and a cup of icing sugar in the bowl of a food processor and grind until nuts are very fine and powdery.
Beat the egg whites in the clean dry bowl of a stand mixer until they hold soft peaks. Slowly add the granulated sugar and beat until the mixture holds stiff peaks.
Sift a third of the almond flour mixture into the meringue and fold gently to combine. Sift in the remaining almond flour in two batches. Be gentle! Don’t overfold, but fully incorporate your ingredients.
Spoon the mixture into a pastry bag fitted with a plain half-inch tip (Ateco #806). You can also use a Ziploc bag with a corner cut off. It’s easiest to fill your bag if you stand it up in a tall glass and fold the top down before spooning in the batter.
Pipe 1" mounds of batter onto baking sheets lined with nonstick liners (or parchment paper). Bake for 5 minutes. Remove the pan from the oven and raise the temperature to 375°F. Once the oven is up to temperature, put the pans back in the oven and bake for an additional 7 to 8 minutes, or lightly colored.
Cool on a rack before filling.