In a large bowl, combine the bread flour, sugar, and kosher salt. Stir well to ensure everything is evenly mixed.

In another bowl, whisk together the warm milk, egg, and sourdough discard until smooth.

Gradually mix the wet ingredients into the dry ingredients until a soft dough forms. Knead the dough by hand or with a stand mixer for about 10 minutes until smooth and elastic.

Add the softened unsalted butter to the dough, kneading until fully incorporated and the dough is elastic.

Place the dough in a greased bowl, cover it, and let it rise at room temperature until doubled in size, approximately 1-2 hours.

Once risen, punch down the dough and roll it out into a rectangle. Spread your desired filling over the dough, then roll it up tightly into a log. Twist the log and place it into a greased loaf pan.

Let the shaped babka rise again for about 30-45 minutes until puffed.

Preheat your oven to 350°F (175°C) and bake the babka for 30-35 minutes or until it’s golden brown on top. Allow to cool before slicing.