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Preheat the oven to 350°F. Lightly grease an 8" square pan; line with parchment or a reusable liner and then lightly grease.
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To make the cake: Working with an electric hand mixer in a large bowl, beat the egg whites and cream of tartar until soft peaks form. With the mixer running, gradually add 2 tablespoons (25g) of the sugar and continue to beat until the mixture is stiff and glossy. Gently tap the 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 a separate large bowl, beat the yolks until well combined. Add the remaining 6 tablespoons (74g) sugar along with the salt and vanilla; 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 to the yolk mixture, beating to combine. Use a flexible spatula to stir in the flour and baking powder; mix until no streaks of flour are visible.
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Add about a third of the egg white mixture to the bowl and stir it in; this will lighten the batter. Then 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 final third of the whites, using gentle folding motions to combine the batter until no white streaks are visible.
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Transfer the batter to the prepared pan and gently smooth the top.
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Bake the cake for 12 to 16 minutes, until a toothpick or thin knife 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 and allow the cake to cool in the pan.
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To make the soak: In a small bowl, 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 in a large bowl, combine the cream, mascarpone, confectioners’ sugar, and vanilla; whisk at low speed until the mixture is combined. Increase the speed to medium-high and whisk until soft peaks form.
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To assemble the tiramisu: Turn the cake layer out of the pan and use a 2 1/2" to 2 3/4" circle cutter to cut 4 layers of cake; this is the perfect size for assembling in two 16-ounce wide-mouth Mason jars or two 10- to 12-ounce standard mugs. (Adjust the size of the cake layers to your mug or jar, as needed.) There will be leftover cake — enjoy as a baker’s snack or cut the scraps, wrap, and freeze for later.
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Scoop 2 heaping tablespoons (about 30g) filling into the bottom of the Mason jar or mug; two rounded tablespoon scoops work well here. Spread the filling into an even layer with the back of a spoon and sift cocoa powder over the entire surface of the filling.
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Take one of the circles of cake and dip both sides into the bowl of soak. Place the soaked cake on top of the cocoa layer. Scoop another 2 heaping tablespoons (about 30g) of filling on top of the cake layer and spread it evenly; sift another layer of cocoa powder over the top of the filling.
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Repeat with a second round of cake, starting with the soak, and then adding cocoa powder and the final 2 heaping tablespoons (about 30g) filling on top. Sift the final layer of cocoa powder over the top of the filling. If using 16-ounce wide-mouth Mason jars, the tiramisu will not reach the top of the jar. If using a mug, the tiramisu will fill the entire mug.
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Repeat the assembly steps with the second jar or mug and remaining cake, filling, soak, and cocoa powder.
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Cover the assembled tiramisus and let them chill in the refrigerator for at least 2 hours, or overnight, before serving.
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Storage information: Store any leftover tiramisu, covered, in the refrigerator for up to 3 days. Serve chilled.
