Offer this chocolate-and-coffee-flavored opera cake as a surprise at the end of the evening, long after the wedding cake has been devoured.
Heat oven to 425 degrees. Coat two rimmed baking sheets with cooking spray. Line with parchment; spray again.
Sift together almond flour and confectioners' sugar in a mixer bowl. Add whole eggs; beat on medium until light, about 3 minutes. Mix in flour until just incorporated. Transfer batter to a separate bowl.
Whisk the egg whites on medium-high in a clean mixer bowl. Gradually add granulated sugar, whisking until stiff, glossy peaks form.
Fold meringue, then butter, into batter with a rubber spatula just until combined. Divide between pans. Bake, rotating pans halfway through, until golden brown, 10 to 12 minutes. Let cool completely, then remove from pans and peel off parchment. Store, tightly wrapped, up to overnight.
To assemble, cut cakes exactly in half crosswise into two equal pieces, ending up with four (set one half aside for another use). Place one cake layer on a baking sheet lined with parchment paper or a cake board. Gently brush 1/3 of coffee syrup over cake; spread. Using an offset spatula, spread 1 cup coffee buttercream over top. Place another layer on top, brush on 1/3 of the syrup, and spread chocolate ganache over top. Add third layer, brush on remaining syrup, and smoothly spread 1 cup buttercream over top. Cake can be refrigerated, covered, up to 2 days.
At least 2 hours before serving, pour glaze over cake; gently spread to completely cover top (don't worry about sides). Refrigerate until glaze is set, about 1 hour.
Trim edges, and cut cake into 1-inch squares using a warm serrated knife (dip in hot water after each cut, and quickly wipe dry, to make sure slices are very clean). Top each square with a small piece of edible gold leaf; to apply, use a small, dry brush to gently pull off pieces from the sheet of gold leaf. Touch the gold leaf onto each cake, which will help remove it from the brush. Cake pieces can be kept in an airtight container at room temperature up to 2 hours.
The accents on the tops of these tiny cakes are flakes of edible 23-karat gold, available in books of 25 sheets (3 3/4 inches square); each sheet will garnish about 15 cake pieces.