Heat oven to 350 degrees F. Grease eight 6-ounce ramekins and place on a baking pan.
In a double boiler, combine chocolate and butter and stir over simmering water until melted. Remove from heat and let cool 5 minutes.
In a bowl, whisk eggs, granulated sugar, salt, and vanilla with an electric mixer until frothy, about 2 minutes. Lightly whisk in the chocolate mixture. Fold in flour until combined.
Divide mixture among prepared ramekins. Place ramekins on baking sheet in the oven and bake 18 to 22 minutes, or until sides are firm and center is slightly soft. Let cool completely.
Place a small heart-shaped stencil or paper with a heart cutout on top of each ramekin. Sprinkle with confectioners' sugar; carefully remove stencil.
