It's easier to separate the eggs while they're still cold. For the egg whites, it's important that the bowl is super clean and grease free. So pour a little bit of white vinegar into the bowl, swirl around and then empty it out. Wipe the bowl with a clean paper towel. Wipe the whisk attachment with vinegar too. Then, separate the eggs, add the egg whites into the super clean bowl and the yolks into a small bowl. Be careful not to mix in any yolks into the whites.
Preheat the oven to 375°F (190°C).
Coat 4 individual serving ramekins (8-oz) generously with softened butter. Sprinkle granulated sugar into the ramekins and rotate the dishes to coat bottoms and sides. Tap out the excess. In a medium heat resistant bowl, add in the chopped chocolates. Microwave for 10-15 seconds intervals, stirring each time, until about 90 percent of the chocolate is melted. At this point, keep stirring the chocolate until the rest of pieces are melted with the remaining heat. You can also melt the chocolate over simmering water, make sure the bowl is not touching the water. Set aside.
In a small saucepan on medium heat, add the butter. Once melted, add in the flour and whisk until combined. While whisking, gradually add in milk. Then add 1 tablespoon of sugar (we'll use the rest for the egg whites) and salt. Whisk to combine. Let the mixture cook on medium heat for 1-2 minutes, until slightly thickened to coat the back of a wooden spoon.
Now while whisking, add the mixture to the melted chocolate. Add in the yolks too and whisk until combined. Then add in vanilla extract and mix. Set aside.
Add cream of tartar into the bowl with egg whites. Start beating the egg whites with a whisk attachment on medium high until frothy. Then, while beating, slowly add in the rest of granulated sugar (2 tablespoons). Once all the sugar is added, continue beating on medium-high until stiff peaks form and it looks glossy.
Using a spatula, add about ¼ of the egg whites into the chocolate mixture and fold. Fold in the remaining egg whites in 2-3 parts. Be careful not to overmix. The batter will be quite fluffy at the end.
Divide the batter between the ramekins, transfer them to a baking sheet and bake in the preheated oven for 15-20 minutes. They're done when the soufflé is puffed up and slightly browned on top, and when gently shaken, it only moves slightly in the center.
Dust the tops with powdered sugar and serve immediately because Soufflé will deflate after cooling down. You can also top them with a scoop of vanilla ice cream or a dollop of whipped cream.