Preheat your oven to 400°F (200°C). Generously grease the inside of four 6-ounce ramekins with the softened butter, using upward strokes to help the souffles rise. Coat the insides entirely with granulated sugar, tapping out any excess.

Place the chopped dark chocolate and instant espresso powder in a medium heat-proof bowl. In a small saucepan, heat the milk until it just begins to simmer, then pour it over the chocolate. Let it sit for 1 minute, then whisk until smooth and glossy.

Stir the vanilla extract and egg yolks into the melted chocolate mixture until well combined. Set this base aside to cool slightly while you prepare the egg whites.

In a large, perfectly clean glass or metal bowl, beat the egg whites, cream of tartar, and salt with an electric mixer on medium speed until frothy. Gradually add the 1/4 cup of sugar, a little at a time, and increase the mixer speed to high.

Continue beating until the egg whites hold stiff, glossy peaks. When you lift the beaters, the peaks should stand straight up without drooping, which usually takes about 3 to 4 minutes.

Gently fold one-third of the whipped egg whites into the chocolate base using a rubber spatula to lighten the mixture. Carefully fold in the remaining egg whites until no white streaks remain, being very careful not to deflate the delicate batter.

Divide the batter evenly among the prepared ramekins, filling them all the way to the top. Level off the tops with a straight edge, like a butter knife, and run your thumb around the inside rim of each ramekin to create a small channel, ensuring an even and straight rise.

Place the ramekins on a baking sheet and bake for 12 to 15 minutes, or until the souffles have risen highly above the rims and the centers are just slightly jiggly. Serve immediately, lightly dusted with powdered sugar if desired, as they will begin to deflate quickly once out of the oven!