1. Prepare the espresso by combining all the ingredients.  Set it aside or store it in the fridge until you are ready to assemble the cake.

For the Sponge Cake:

1. Line a 9 x 13 baking sheet and spray it well.  Set it aside.  Preheat oven to 350 degrees F.
2. In a large mixing bowl, beat eggs and sweeter until eggs lighten in colour (about 5 minutes).  Add in cream of tartar and vanilla and beat an additional minute.
3. In a separate bowl, combine the flour and whey, xanthan gum, baking powder, baking soda and salt.  Whisk to incorporate the ingredients.
4. Fold the dry ingredients into the egg mixture, making sure all the dry ingredients are incoporated into the egg mixture, being careful not to deflate the eggs.
5. Spread the mixture into the baking tray and bake for 10-12 minutes or until the top of the sponge is beginning to brown.
6. Remove the sponge cake from the oven and allow it to cool slightly.  Once slightly cooled, transfer the cake (including the parchment paper) to a cooling rack.  Allow the cake to fully cool.

For the Mascarpone Layer

1. In the bowl of a stand mixer, fitted with the whisk attachment, beat the yolks at low speed until just combined.  Add the sweetener and salt and beat at medium-high speed until pale yellow, about 2 minutes, scraping down the bowl with a rubber spatula once or twice.  Add 3 tablespoons of rum and beat at medium speed until just combined, 20-30 seconds. Scrape the bowl down again.  Add the mascarpone and beat at medium speed until no lumps remain, 30-45 seconds, scraping down the bowl once or twice.  Transfer the mixture to a large bowl and set it aside.
2. In a separate bowl, beat the heavy cream and vanilla extract together on medium-high speed for 3-4 minutes or until medium peaks form.
3. Fold the whipped cream into the mascarpone mixture.  Taste the mixture and add additional sweetener if desired.

Assemble the Cake:

1. Cut the sponge cake in half in order to fit on the bottom of an 8 x 8 pan.  You will have a few leftover pieces of cake which you can use to fill in any gaps between the cake and the sides of the pan.
2. Brush the espresso on one side of the cake, adding a good soak, and place the espresso-soaked side down into the bottom of an 8 x 8 pan.  Soak the top of the cake with additional espresso
3. Spread 1/2 of the mascarpone on the cake.  Using a sieve, dust 2 tablespoons of unsweetened cocoa powder over the mascarpone.
4. Repeat the layer by soaking one side of the remaining cake with espress.  Flip the cake onto the mascarpone, making sure that the espresso side is facing down.  Soak the top side of the cake with espresso.  Spread the remaining mascarpone over the cake and dust with the remaining 2 tbsp of cocoa powder.  Refrigerate for at least 6 hours or overnight.
5. If desired, add white chocolate curls or grated chocolate over the top layer right before serving.