For the cake: Adjust rack in lower third of oven, preheat to 350 degrees. Sift the flour, 3/4 cup of the sugar and the salt onto a sheet of wax paper, set aside.
In a small bowl using an electric hand mixer, beat the egg yolks with 1/4 cup of the sugar until the mixture is thick and pale in color. Add the water and continue to beat until thickened, about 4 minutes. Whisk the egg whites in the bowl of a heavy-duty mixer just until frothy. Add the cream of tartar and continue to whisk until soft peaks form. Add the remaining 1/2 cup sugar in a steady stream, whisking until thicker, stiffer, glossy peaks form, about 2 to 3 minutes. Whisk in the vanilla, lemon juice and lemon zest.
Pour the yolk mixture over the beaten egg whites. Fold the two mixtures together with a rubber spatula. Using a metal spatula, scoop up a third of the flour mixture and sprinkle it over the surface, fold in with the rubber spatula. Repeat 2 more times just until the ingredients are incorporated. Gently pour batter into an ungreased 10-inch round tube pan with removable bottom (like an angel food pan). Level the surface with the rubber spatula.
Bake for 50 to 55 minutes or until the top springs back slightly when lightly touched, and sounds spongy. Remove the cake from the oven and invert the pan over a long-necked bottle to cool for about 45 minutes. To remove the sponge from the pan, carefully slip a flexible metal spatula down one side of the pan and slowly trace the perimeter to release the cake. When the sides are free, push up on the removable bottom to release the cake completely. Tilt the cake, with the bottom still attached, and gently tap the bottom against the counter to loosen the cake. Rotate the cake, tapping a few more times, until it appears free. Cover the cake with a rack and invert, remove the bottom of pan. Let cool to room temperature.
Coffee crunch topping: Generously oil a large baking sheet. Sift the baking soda onto a small sheet of wax paper, set nearby. Combine the coffee, sugar and corn syrup in a deep, heavy 4-quart saucepan. Place over medium-low heat, stirring occasionally, until the sugar dissolves. When the mixture is clear and it begins to boil, increase the heat to medium-high and cook until the mixture reaches 290 degrees on candy thermometer. Toward the end of cooking, around 270 to 280 degrees, stir occasionally to prevent the mixture from scorching and becoming too foamy.
Remove from heat and stir in baking soda. The mixture will foam up fiercely. While mixture is still foaming, pour it out onto the oiled baking sheet. Do not spread, just let it cool undisturbed for at least 1 hour.
Break or crush into very small pieces by placing between 2 sheets of wax paper and tapping or rolling with rolling pin. Store in airtight container.
Frosting: Combine cream, sugar and vanilla in a bowl and beat until the cream holds soft peaks.
To assemble: Slice the cooled cake into 3 equal layers using a serrated knife. Spread whipped cream between each layer, carefully stacking the layers. Spread the remaining whipped cream over the top and sides of the cake. Refrigerate. Just before serving, generously sprinkle top and sides with coffee crunch.