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Preheat the oven to 350°F. Lightly grease two 8" square pans; line them with parchment and grease the parchment. If you only have one 8" square pan, see "tips," below for details.
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To make the cake: In the bowl of a stand mixer fitted with the whisk or working with an electric hand mixer and a large bowl, combine the egg whites and cream of tartar. Beat until soft peaks form. Gradually add 3 tablespoons (37g) of the sugar, continuing to beat until the mixture is stiff and glossy. Set aside. (If you only have one stand mixer bowl, transfer the whites to another bowl so that you can use the mixer bowl for next step.) Gently tap the whisk or beaters against the side of the bowl to knock off as much of the whites as possible; no need to clean them before the next step.
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In another large bowl or stand mixer bowl, beat the yolks until well combined. Add the remaining sugar (1/2 cup plus 1 tablespoon, 112g) and the salt, and beat until the mixture comes together and thickens, 3 to 5 minutes. When you stop beating, the mixture should fall in ribbons as you lift the whisk/beaters out of the bowl.
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Add the water and vanilla to the yolk mixture, beating to combine.
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Stir in the flour and baking powder, beating just to combine.
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Stir in about 1/3 of the egg white mixture to the rest of the batter to lighten it. Then use a flexible spatula to gently fold in another third of the egg whites, this time being more careful not to deflate the whites. Once it is mostly incorporated, fold in the remaining third of the whites, using gentle folding motions to combine the batter until no streaks of white are visible.
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Divide the batter among the prepared pans, portioning a generous 1 3/4 cups (about 205g) per pan; the layer will be thin. Gently smooth the top.
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Bake the cakes for 12 to 16 minutes, until a toothpick inserted into the center comes out clean. Remove from the oven and set on a rack. Loosen the edges with a spatula or table knife.
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To make the soak: In a small bowl or liquid measuring cup, combine the boiling water, sugar, and espresso powder. Stir until the sugar is completely dissolved, then stir in the alcohol.
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To make the filling: In the bowl of a stand mixer or working with an electric hand mixer and a large bowl, combine the cream, mascarpone, and vanilla and whisk at low speed until the mixture is smooth.
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In a small bowl, combine the confectioners’ sugar and Instant ClearJel, and slowly add to the cream mixture. Increase the speed to medium-high and whisk until soft to medium peaks form.
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To assemble the tiramisu: Turn both cake layers out of the pan.
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Use a pastry brush to apply about 1/4 cup (roughly 57g) of the soak across the top of each cake layer (using 1/2 cup total). Allow the soak to fully absorb into the cake. Place one layer, soaked-side down, into a clean 8" square pan.
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Brush another 1/4 cup (about 57g) of the soak onto the plain, unsoaked side of the cake layer that was just placed in the dish.
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Spread half the filling, about 1 3/4 cups (roughly 285g), over the top of the soaked cake layer; a small offset spatula is a helpful tool here.
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Sift 1 tablespoon of the cocoa powder over the entire surface of the filling.
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Gently place the second layer of cake, soaked-side down, over the cocoa powder.
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Brush the remaining soak (about 1/4 cup) onto the plain, unsoaked side of the cake layer that was just placed in the dish.
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Spread the remaining filling over the top of the soaked cake layer, then finish by sifting the remaining 1 tablespoon cocoa powder over the top.
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Cover the assembled tiramisu and let it chill in the refrigerator for at least 2 hours, or overnight, before slicing and serving.
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Storage information: Serve cold and store any leftover tiramisu, covered, in the refrigerator for several days.