Butter a 9-by-13-inch baking pan. Preheat an oven to 350 degrees F.
In the top pan of a double boiler or a heatproof bowl set over (not touching) simmering water in a pan, combine the butter and chocolate and stir until melted and smooth. Remove from the heat and let cool.
In a bowl, using an electric mixer set on medium speed, beat together the eggs and granulated sugar until the mixture is thick, pale yellow and holds a trail for 3 or 4 seconds when the beaters are lifted from the batter, 3-4 minutes.
Stir in the vanilla and the cooled butter-chocolate mixture until no streaks remain; do not overmix. Using a rubber spatula, fold in the flour, baking powder and salt until just blended. Fold in the almonds.
Pour the batter into the prepared pan. Bake until a toothpick inserted into the center comes out with some wet crumbs attached, 25-30 minutes.
Transfer to a rack and let cool in the pan. Cut into 2-inch squares. Just before serving, using a fine-mesh sieve or sifter, dust lightly with the confectioners’ sugar.
Recipe reprinted by permission of Weldon Owen. All rights reserved.