In a large mixing bowl, combine the flour, sugar, salt, and instant yeast. Mix well to ensure even distribution.

Add the cold, cubed butter to the flour mixture. Using a pastry cutter or your fingers, work the butter into the flour until the mixture resembles coarse crumbs.

In a separate bowl, whisk together the sour cream, milk, egg, and vanilla extract. Pour this into the flour mixture and stir gently until just combined.

Turn the dough out onto a lightly floured surface and knead it gently until smooth, about 1-2 minutes. Wrap the dough in plastic wrap and chill in the refrigerator for at least 1 hour.

Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.

In a small bowl, mix together the diced apples, cinnamon, brown sugar, and walnuts if using.

Roll out the chilled dough into a rectangle, about 1/4 inch thick. Spoon the apple mixture down the center of the dough.

Cut slits along the sides of the dough about 1 inch apart. Fold the flaps over the filling to create a braided effect.

Bake for 25-30 minutes or until golden brown. Allow to cool slightly.

While cooling, whisk together the powdered sugar, 2 tablespoons of milk, and 1/2 teaspoon of vanilla until smooth.

Drizzle the glaze over the kringle once cooled slightly.