Preheat oven to 375°F (190°C) with a rack on the lowest level.

Prepare 8 individual soufflé molds by brushing with softened butter.

Dust the inside of the molds with granulated sugar, ensuring even coverage and removing excess.

Create a roux by melting butter in a saucepan over medium heat.

Whisk in flour and stir continuously for 3 minutes until smooth.

Remove from heat and set aside.

Heat milk in a separate saucepan over low heat until simmering.

Whisk in both semisweet and unsweetened chocolate until melted and smooth.

Add sugar, vanilla extract, and rum, whisking until fully incorporated.

Gradually pour the chocolate mixture into the roux, returning to low heat.

Whisk constantly for 5-7 minutes until the mixture thickens and begins to bubble.

Remove from heat and whisk in egg yolks one at a time until fully combined.

Transfer the chocolate mixture to a large mixing bowl.

In a separate bowl, combine egg whites, sugar, and cream of tartar.

Whip with an electric mixer on medium-high speed until soft peaks form.

Gently fold 1/4 of the meringue into the chocolate mixture.

Fold in half of the remaining meringue, being careful to maintain airiness.

Fold in the last of the meringue gently until just combined.

Spoon the mixture into the prepared soufflé molds, filling them 3/4 full.

Bake for 12-14 minutes, or until the soufflés have risen and browned.

Dust with powdered sugar and serve immediately.