Wipe the metal bowl of an electric stand mixer, whisk attachment and a whisk with vinegar or lemon juice and a paper towel to remove any trace of grease.
Fill a saucepan with 1 inch of water and bring to a boil, then reduce to a simmer. Place a metal mixing bowl over the pot of simmering water (bowl should fit snugly but should not touch the water) and add the egg whites and sugar. Stir constantly with a whisk until the temperature reaches 140 degrees on a candy thermometer or until all the sugar has dissolved and the mixture is foamy and the egg whites are hot. Remove from heat.
Beat egg whites and sugar on high speed with a whisk attachment until the bottom of the bowl is completely neutral, about 10 minutes. Switch to the paddle attachment and turn mixer on low speed.
Begin slowly adding the butter pieces and mix until the texture is smooth. If the buttercream curdles, keep mixing, it'll come back together.
Add the vanilla bean paste and a pinch of salt and mix. Add half of the espresso mixture and taste, adding more if you like. Begin adding the amaretto a tablespoon at a time, tasting as you go. Do not add more than ¼ cup or the buttercream may break.
You can store buttercream in the fridge in an airtight container for 3 days, or in the freezer for up to 6 weeks. Assembly When cakes are cool, cut out rounds with a 4 oz ball jar (or something similar). The cake recipe is pretty delicate so don't worry if the rounds break, they can be pressed into the jars and will look and taste just the same. Press one layer in, pipe or spoon buttercream filling, then top with the second round of cake and pipe or spoon on buttercream and top with freshly grated cinnamon, if desired. After you've punched out all your rounds, piece together unused cake into jars and repeat.
Recipe by oh honey bakes at https://ohhoneybakes.com/dark-chocolate-and-amaretto-spoon-cakes/