Melt the dark chocolate in a heatproof bowl set over a saucepan of simmering water, stirring occasionally until smooth. Remove from heat and let cool slightly.

In a large mixing bowl, beat softened butter and granulated sugar together until light and fluffy, about 3 minutes.

Add egg yolks one at a time, beating well after each addition.

Stir in melted chocolate, strong coffee, and vanilla extract until well combined.

In a separate clean bowl, add a pinch of salt to egg whites and beat until stiff peaks form.

Gently fold beaten egg whites into chocolate mixture in three additions, being careful not to deflate the mixture.

Line a loaf pan with plastic wrap, leaving enough overhang to cover the top.

Pour chocolate mixture into prepared pan, smoothing the top with a spatula.

Fold overhanging plastic wrap over the top to cover.

Refrigerate for at least 4 hours or overnight until firm.

Heat heavy cream in a small saucepan until it begins to simmer. Remove from heat.

Add chopped dark chocolate and let sit for 1 minute.

Stir until smooth and glossy. Let cool slightly.

Remove marquise from refrigerator and unwrap plastic wrap.

Invert onto a serving platter and peel off plastic wrap completely.

If using ganache, pour over the top, allowing it to drip down the sides.

Dust with cocoa powder and garnish with fresh berries and whipped cream if desired.

Slice with a hot knife — dip knife in hot water and wipe dry before each cut for clean slices.