Make the meringues in advance to free up your oven for the sponge.
1 1/4 cups icing sugar
1 cup finely ground sliced, blanched almonds
6 tablespoons egg whites
Pinch of salt
1/4 cup sugar
3 large egg whites
1 cup sugar
1 cup unsalted butter, at room temperature, cut into pieces
To Make Filling:
1. In the bowl of an electric mixer, whisk egg whites and sugar. Set mixer bowl over a saucepan of simmering water and heat mixture, whisking often, until it feels warm to the touch and sugar is dissolved, 3 to 5 minutes.
2. Transfer bowl to the mixer, and whip on high speed until mixture is stiff and shiny, 3 to 5 minutes. Add butter, one piece at a time, and continue mixing until butter is thoroughly incorporated.
To Make Macarons:
1. Preheat oven to 350 degrees. In a medium bowl, whisk together icing sugar and ground almonds.
2. With an electric mixer, whip egg whites with salt on medium speed until foamy. Increase speed to high and gradually add sugar. Continue to whip until stiff glossy peaks form.
3. Using a spatula, gently fold in the icing sugar mixture.
4. Spoon mixturte into piping bag, Pipe 1-inch disks onto prepared baking sheets, leaving 2 inches between cookies. The batter will spread a little. Let stand at room temperature until dry, and a soft skin forms on the tops of the macarons - about 15 minutes.
5. Bake with the door of the oven slightly ajar, until the surface of the macarons is completely dry, for about 15 minutes. Remove baking sheet to a wire rack and let the macarons cool completely on the baking sheet. Gently peel off the paper.
6. Fill a pastry bag with the filling. Turn macarons so the flat bottoms face up. On half of them, pipe 1 teaspoon filling. Sandwich these with the remaining macarons, pressing slightly to spread the filling to the edges. Refrigerate until firm.