In a separate bowl, whisk the egg whites with granulated sugar until stiff peaks form, then gently fold them into the egg mixture.
Stir in the melted butter and mix until just combined.
Divide the batter between the prepared baking sheets and spread evenly. Bake for 10-12 minutes or until golden. Let the sponge cool completely.
Prepare the Coffee Syrup:
In a small saucepan, combine the water, sugar, and instant coffee granules. Bring to a boil, stirring until the sugar dissolves. Remove from heat and let cool.
Make the Coffee Buttercream:
In a small saucepan, combine the sugar and water, and bring to a boil until it reaches 240°F (115°C) on a candy thermometer.
In a separate bowl, beat the egg yolks until thick and pale, then slowly pour in the hot syrup while continuing to beat.
Add the softened butter, one piece at a time, beating until smooth and creamy. Dissolve the coffee granules in a small amount of hot water, then mix into the buttercream.
Prepare the Chocolate Ganache:
Place the chopped chocolate in a heatproof bowl. Heat the cream in a saucepan until it begins to simmer, then pour it over the chocolate.
Let it sit for a minute, then stir until smooth. Allow the ganache to cool slightly before using.
Assemble the Opera Cake:
Place one Joconde sponge layer on a serving plate and brush generously with coffee syrup.
Spread half of the coffee buttercream over the sponge, then top with another sponge layer. Repeat the process, adding the remaining buttercream and another sponge layer.
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