Beat egg whites and cream of tartar in mixer bowl with whisk attachment on high speed until foamy. Beating constantly, add 1/3 cup of the granulated sugar, 2 tbsp. at a time, beating after each addition until sugar is dissolved before adding the next. (Rub a bit of mixture between thumb and forefinger; it should feel completely smooth.) Continue beating until whites are glossy and stand in soft peaks.
Beat egg yolks in clean mixer bowl on high speed until thick and lemon-colored, 3 to 5 minutes. Gradually beat in remaining 1/3 cup granulated sugar until pale yellow and sugar is dissolved. Beat in almond extract, vanilla and salt.
Mix almonds and flour in small bowl; sprinkle evenly over egg whites. Add yolk mixture. Fold gently but thoroughly until color is uniform and no streaks of white remain. Do not stir. Spread in prepared pan.
Sprinkle a clean kitchen towel with a dusting of powdered sugar. Loosen cake from sides of pan with knife. Invert cake onto towel; carefully peel off waxed paper. Trim all cake edges with serrated knife. Brush syrup evenly on hot cake. Roll up cake, starting from short end and rolling towel in with cake. Cool wrapped cake roll, seam-side down on wire rack.
Carefully unroll cooled cake; remove towel. Spread evenly with about 1 cup frosting; reroll cake. Place seam-side down on platter. Cover with about 1 cup frosting, using small spatula to create tree-bark effect.
Make ahead tip: Cooled unfilled cake roll can be frozen, wrapped well in aluminum foil or plastic wrap. Defrost wrapped cake at room temperature, about 1 hour, before filling.
Sponge cakes rely entirely on beaten eggs for their structure and leavening. Good techniques for beating eggs with sugar are important.
Keep the yolks separate from the whites. Fat from egg yolk will prevent egg whites from beating up properly. When separating eggs, take care that no yolk gets in the whites. To avoid an accident, separate each egg white into a cup or small bowl before transferring it to the mixer bowl. Discard any white that has even a speck of yolk in it.
Equipment: Beaters and bowl should be spotlessly clean. Any residue of fat will prevent egg whites from beating up properly. Use a stainless steel or glass bowl. Plastic bowls can retain a film of grease.
Egg temperature: It’s easiest to separate eggs cleanly when they are refrigerator cold. However egg whites whip up to greater volume when they’ve had a chance to warm up a bit, 20 to 30 minutes. Always begin by separating the eggs. Let the whites stand at room temperature while you prepare the baking pan, equipment and other ingredients.
Add sugar gradually. For optimum volume and smoothest texture, sugar should be added gradually, beginning only after the egg whites or yolks have been beaten to the proper stage. Adding some or all of the sugar before beginning to beat will result in less volume.
To check if sugar is dissolved: After each addition, eggs should be beaten until sugar has dissolved before adding more. To test, rub a bit of mixture between thumb and forefinger. If sugar is dissolved, it will feel completely smooth. If it feels grainy or sandy, continue beating.
What’s a soft peak? The egg white mixture should be beaten until it appears glossy and stands in soft peaks that curl at the tips when the beater is lifted. If the peaks stand straight and tall (stiff peaks), it has been overbeaten.
Gentle folding is the key to maintaining volume. Combining heavier mixtures with beaten egg whites can knock the air out of them. Add the flour and egg yolk mixture to the beaten whites, not vice versa. Fold with a light touch, rather than stirring. Using a rubber spatula, start with a downward stroke into the bowl, continue across the bottom, up the side and over the top of the mixture. Come up through the center every few strokes and rotate the bowl often as you fold. Fold just until no streaks of flour remain.