Preheat the oven to 300 degrees. Draw three 8-inch circles on parchment paper – you will need 2 pieces of parchment. Turn the paper over and place on two baking sheets.
Beat the egg whites, cream of tartar, and salt on medium-high speed until foamy. Increase speed to high then gradually add the sugar. Beat until stiff peaks form.
Whisk the almond flour and cocoa powder together until there are no lumps. Add to the egg whites and beat for 30 seconds on low speed. Using a spatula, make sure everything is fully combined.
Dollop one third of the mixture in the middle of each drawn circle, then spread into an even layer with an offset spatula. Bake for 45 minutes.
Place the dacquoise, still on the parchment paper, on wire racks to cool completely. When they are completely cooled, gently peel the paper off the layers. Place one dacquoise layer on a serving platter, spread with half of the buttercream frosting, place another dacquoise layer on top, spread the rest of the butter cream on top in an even layer, and top with the remining dacquoise layer. Dust with powdered sugar.
Can be served immediately or refrigerated for up to one day.
Mocha Buttercream Frosting:
Beat the shortening until fluffy.
Sift the powdered sugar and cocoa powder together. Add to the shortening and beat. Beat in the vanilla then the coffee, 1 tablespoon at a time, until it reaches a thick yet easily spreadable consistency. Can be used immediately or stored in the refrigerator, covered, for several days.