1. Preheat the oven to 400 F (205°C). Generously butter two 6-oz ramekins and dust them with cocoa powder, then set aside.

2. Combine the butter and chocolate in a small saucepan and melt together over low heat, stirring occasionally until completely smooth. Take off the heat and stir in the vanilla.

3. In a separate bowl using electric beaters or in the bowl of an electric mixer fitted with the whisk attachment, beat the egg on medium speed until foamy, then slowly add the sugar and salt. Beat on high for another 5 - 10 minutes, until the mixture is very thick and pale yellow in colour.

4. Pour the egg mixture into the melted chocolate and butter, then sift the flour directly over top. Gently fold everything together using a spatula, just until incorporated.

5. Divide the batter between the two prepared ramekins, smoothing the tops as best you can with the spatula. Place the ramekins on a rimmed baking sheet and bake for 10 - 13 minutes, or until the cakes have puffed slightly and jiggle just a bit in the centre when gently shaken.

6. Remove from the oven and immediately run a knife around the edges of each cake. Invert the ramekins onto serving plates and let them sit for about 1 minute to allow the cakes to release from the ramekins, then lift them off the cakes. Dust with icing sugar (if using) and serve immediately. We recommend serving them with a dollop of whipped cream and fresh berries.