Grease the ramekins with butter and dust with granulated sugar. This ensure that the cakes come out of ramekins easily when inverted onto plates. Set aside.

Put the chocolate and butter in a heatproof bowl.

Place the bowl over a saucepan of simmering water and let the chocolate and butter melt. Stir occasionally.

Let the chocolate mixture cool.

In the bowl of a stand mixer, whisk the eggs, salt, sugar and vanilla extract until doubled in volume and the egg mixture become pale.

Pour the egg mixture over the chocolate and add the flour.

Gently mix everything until well combined.

Pour the chocolate batter in the ramekins and place the ramekins on a baking sheet.

Bake in 425°F preheated oven for 10 to 12 minutes or until the edges and tops are firm but the centers are still soft.

Wait 1 to 2 minutes, then run a knife around the edges of the cakes, then invert each cake on a plate.

Dust with powdered sugar (optional) and serve immediately.