Divide the oven into thirds by positioning a rack in the upper third of the oven and the lower third of the oven.
Preheat the oven to 425?F. (220?C).
Line two 12½ x 15½- inch (31 x 39-cm) jelly-roll pans with parchment paper and brush with melted butter.
In the bowl of a stand mixer fitted with the whisk attachment (or using a handheld mixer), beat the egg whites until they form soft peaks. Add the granulated sugar and beat until the peaks are stiff and glossy. If you do not have another mixer bowl, gently scrape the meringue into another bowl and set aside.
If you only have one bowl, wash it after removing the egg whites or if you have a second bowl, use that one. Attach the paddle attachment to the stand mixer (or using a handheld mixer again) and beat the almonds, icing sugar and eggs on medium speed until light and voluminous, about 3 minutes.
Add the flour and beat on low speed until the flour is just combined (be very careful not to overmix here!!!).
Using a rubber spatula, gently fold the meringue into the almond mixture and then fold in the melted butter. Divide the batter between the pans and spread it evenly to cover the entire surface of each pan.
Bake the cake layers until they are lightly browned and just springy to the touch. This could take anywhere from 5 to 9 minutes depending on your oven.
Place one jelly-roll pan in the middle of the oven and the second jelly-roll pan in the bottom third of the oven.
Put the pans on a heatproof counter and run a sharp knife along the edges of the cake to loosen it from the pan. Cover each with a sheet of parchment or wax paper, turn the pans over, and unmold.
Carefully peel away the parchment, then turn the parchment over and use it to cover the cakes. Let the cakes cool to room temperature.