Step 1

Preheat the oven to 375°F (190°C) and set an oven rack in the middle position.

Step 2

Grease four 6-oz (180-ml) soufflé molds generously with the softened butter, making sure to coat the bottoms and sides evenly. Sprinkle the granulated sugar into one greased soufflé cup, rotating it to coat the sides evenly while pouring the excess into the next cup. Repeat until all cups are coated, then discard any unused sugar.

Step 3

In a medium saucepan over medium heat, melt the butter. Add the flour and whisk continuously for about 1 minute until bubbling and smooth. Whisk in the cocoa powder, then add the milk, liqueur (if not using liqueur, replace with more milk), vanilla, and salt. Cook the mixture, stirring constantly, until it gently bubbles, thickens, and becomes smooth, about 2 minutes.

Step 4

Remove the saucepan from the heat and add the chocolate. Whisk until the chocolate is completely melted and the mixture is smooth. Quickly whisk in the egg yolks, one at a time, until the mixture is smooth. Set aside.

Step 5

In the bowl of an electric mixer fitted with the whisk attachment, beat the egg whites on medium-high speed until soft peaks form, about 1 minute. Gradually add the granulated sugar and continue beating until medium peaks form (the peaks should hold but droop slightly), about 2½ minutes more. Be careful not to overbeat; the mixture should not be too stiff or dry, as that will make it difficult to incorporate into the chocolate mixture.

Step 6

Add one-third of the beaten egg whites to the chocolate mixture. Using a whisk, mix gently until evenly combined to lighten the mixture. Then, add the lightened chocolate mixture to the bowl of remaining beaten egg whites and using a rubber spatula, fold gently until fully incorporated.

Step 7

Divide the mixture evenly among the prepared molds, filling them about ⅛ inch from the top (you can pour carefully or use a ladle). Wipe the rims clean with your fingers. Place the filled molds on a baking sheet and transfer to the oven. Bake for 15 minutes, or until the soufflés have risen, are set on the outside, and wobble gently when you shake the pan. Some soufflés may rise perfectly straight, while others might be a bit uneven—that's completely normal, so don’t worry! They’ll all taste delicious.

Step 8

Using two spatulas, carefully lift the hot soufflé cups to serving plates. Dust with powdered sugar, if using, and serve immediately.