Gather ingredients so you have everything close at hand.
Prepare the Dough
In large mixing bowl, combine sourdough starter, water, 2 tablespoons diastatic malt powder, salt, instant yeast, and flours. (Depending on the hydration of your sourdough, you may not need all three cups of all-purpose flour. Add 2 1/2 - 3 cups, as needed.) Mix together until combined. Knead to form a stiff dough, about 4-5 minutes.
Place dough in lightly greased bowl and cover. Allow to rise for one hour. Gently deflate and allow to rise for another 30 minutes.
Form and Bake the Bagels
Transfer the dough to work space and divide into 12 pieces. Roll each into a smooth, round ball. Cover and allow to rise another 30 minutes. Meanwhile, fill a large saucepan about half full with water. Add remaining diastatic malt powder (or brown sugar) and baking soda to the water. Bring to a gentle boil. Preheat oven to 425 degrees.
Using a finger, poke a hole through the center of each ball and stretch until the hole is about 2 inches wide.
Transfer the bagels, four at a time, to the simmering water, maintaining a gentle boil with heat as needed. Cook the bagels for 2 minutes. Flip over and cook an additional minute. Strain bagels from water and place them on a baking sheet lined with parchment paper. Repeat with remaining bagels.
If desired, brush egg wash (1 egg + 2 tablespoons water) over bagels and sprinkle with sesame seeds, poppy seeds, or other bread toppings, such as Everything Bagel Topping.
Bake for 25 minutes until golden brown. Cool on wire racks. Serve warm with cream cheese. Makes 1 dozen chewy bagels.