1-First, melt the unsalted butter and chopped semi-sweet chocolate bar together in a microwave-safe bowl or over a double boiler, stirring every 20 seconds, then let it cool slightly.

2-Next, whisk the egg yolks, vanilla extract, and salt into the chocolate mixture and set it aside.

3-In a clean bowl, beat the egg whites and cream of tartar until soft peaks form, then gradually add the granulated sugar in three additions, beating until stiff, glossy peaks form without over-beating.

4-Carefully fold the egg whites into the chocolate mixture in three portions, gently combining each addition to avoid deflating the batter.

5-Refrigerate the batter for 5-10 minutes while the oven preheats.

6-Position the oven rack in the lower third and preheat the oven to 400°F (204°C). Prepare four 6-ounce straight-sided ramekins by buttering them and coating them thoroughly with granulated sugar, then place them on a baking sheet.

7-Spoon the batter evenly into the ramekins, smooth the surface, and run a knife or thumb gently around the rim to create a channel.

8-Place the baking sheet with ramekins in the oven, immediately reduce the temperature to 375°F (191°C), and bake for 13-14 minutes until the edges are set and the center jiggles slightly.

9-Avoid opening the oven door frequently to prevent collapse, and serve immediately with optional toppings, as the souffles settle quickly.

10-Leftovers can be refrigerated for up to 3 days.