Lightly spray an 8½ or 9-inch loaf pan with vegetable cooking spray. Line the pan with a double layer of plastic wrap allowing it to hang over the sides. Smooth out the plastic wrap as best you can. Place the pan in the freezer until needed.
Melt the chocolate over a saucepan of gently simmering water (bain-marie). Don’t let the bowl touch the water. Set the fully melted chocolate aside to cool. Leave the water on the stove-top; you’ll need it again in the next step.
In a large metal bowl lightly whisk the egg yolks. Dissolve the espresso powder in the water and add to the egg yolks. Add the sifted cocoa powder, butter, vanilla and sugar.
Set the bowl over the bain-marie. Whisk constantly and cook the egg mixture until it reaches 160°F on an instant read thermometer, about 5-6 minutes.
Remove from the heat and beat with an electric mixer until slightly thickened and cooled slightly, about 5 minutes. Add the melted chocolate and stir until blended.
In a separate clean bowl beat the whipping cream until creamy, soft peaks form.
Gently fold one-third of the whipped cream into the chocolate mixture to lighten. Using a rubber spatula gently fold the remaining whipped cream into the chocolate mixture until no white streaks remain. Do not over-mix or deflate.
Spoon the marquise mixture into the prepared pan. Smooth the top and push the mixture into the corners. Fold the extra plastic wrap over the surface to cover. Freeze until firm, about 6 hours or overnight.