01 - Preheat the oven to 400°F. Place a baking tray in the oven to heat.
02 - Grease two 6-ounce ramekins with softened butter, coating the bottom and sides thoroughly. Sprinkle with granulated sugar, tapping out any excess.
03 - In a heatproof bowl set over barely simmering water, melt the chocolate with 1 tablespoon butter and the milk, stirring until smooth. Remove from heat and let cool slightly.
04 - Whisk in the egg yolk and vanilla extract until well combined.
05 - In a clean bowl, beat the egg whites and salt until soft peaks form. Gradually add the sugar and continue beating until glossy, stiff peaks form.
06 - Gently fold one-third of the whipped egg whites into the chocolate mixture to lighten it. Carefully fold in the remaining egg whites until just incorporated—do not overmix.
07 - Divide the mixture evenly between the prepared ramekins, filling each almost to the top. Run your thumb around the inside edge to help them rise evenly.
08 - Place ramekins on the preheated baking tray and bake for 12–15 minutes, or until the soufflés are puffed and just set but still slightly wobbly in the center.
09 - Remove from oven, dust with powdered sugar, and serve immediately with fresh berries if desired.