Don't overwork the dough. You can use a cookie scoop for easy portioning in Step 4, if desired.
Combine yeast and warm water in a bowl; let stand 5 minutes.
Beat butter at medium speed with an electric mixer until creamy; gradually add 1 1/3 cups sugar and 2 1/2 tsp. salt. Add eggs, 1 at a time, beating just until blended after each addition. Stir in yeast mixture.
Add 8 1/2 cups flour to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed just until blended, stopping to scrape bowl as needed. Place dough in a well-greased bowl, turning to grease top. Cover with plastic wrap, and chill 8 to 24 hours.
Shape dough into 35 (2-inch) balls (about 1/4 cup per ball), using floured hands. Place 1 1/2 inches apart on 2 lightly buttered baking sheets. Cover and let rise in a warm place 1 hour or until doubled in bulk.
Preheat oven to 375°. Stir together blueberries and preserves. Combine 1/3 cup flour and next 2 ingredients with a pastry blender until crumbly. Press thumb into each dough ball, forming an indention; fill each with 1 Tbsp. berry mixture. Sprinkle with flour mixture. Bake 20 to 25 minutes or until golden.
To make ahead: Prepare through Step 3; chill 8 to 24 hours. Proceed as directed in Steps 4 and
Mango Kolaches: Prepare as directed, substituting 2 cups chopped fresh mango for blueberries and 1/3 cup peach preserves for blueberry preserves.