Preheat oven to 425°.
Spread 1/2 teaspoon butter inside each of 4 (8-ounce) ramekins; sprinkle evenly with granulated sugar.
Combine remaining 1 teaspoon butter, flour, brown sugar, egg yolks, and yogurt in a medium bowl, stirring mixture well with a whisk.
Place cream of tartar and egg whites in a large bowl; beat with a mixer at high speed until stiff peaks form. Gently fold one-fourth of egg white mixture into yogurt mixture; gently fold in remaining egg white mixture. Divide the mixture evenly among prepared ramekins.
Place ramekins on a baking sheet. Bake at 425° for 10 minutes or until puffy and set.
Combine spiced apple and caramel syrup; spoon 1 tablespoon apple mixture into center of each hot soufflé. Serve immediately.