Combine flour, corn starch, and salt in a large mixing bowl.
Cut in the cream cheese and unsalted butter with a fork until completely combined and a smooth dough forms.
Cut the dough into 3 equal pieces, and form in 3 by 6-inch rectangles and wrap in plastic wrap.
Refrigerate for at least 1 hour, up to overnight.
Kolaczki Filling(s)
While the dough is resting, combine even amounts of each flavor of fruit spread or jam, corn starch, lemon juice (if using), and kosher salt in individual bowls and mix until combined. I typically use four flavors of St. Dalfor brand fruit spread, and divide the jam ingredients into four equal parts.
Reserve until ready to shape the cookies. All of the flavors of filling can be made a head of time. Cover and refrigerate until needed.
Shaping and Baking
Preheat the oven to 350°F.
Heavily flour a work surface. Remove one of the dough rectangles from the fridge, and roll into a 12 by 9-inch rectangle until it's about ⅛-inch thick. Trim the edges of the dough with a sharp knife or a bench scraper to keep the dough square as needed.
Cut the dough into 3”x3” squares. Using a tape measure or ruler will help to keep your cookies even.
Place 1 teaspoon of the prepared filling(s) of your choice in the middle of each dough square.
Working one square at a time, pick up a square and lightly wet two opposing corners and pinch their faces together tightly. Fold the point down over the cookie and lightly press into the dough to ensure the cookie doesn’t come apart while baking. Transfer to a parchment paper-lined baking sheet. Repeat with the remaining dough squares.
Bake for 15 to 17 minutes until lightly golden brown, and the filling has lightly melted.
Remove and allow to cool approximately 5 minutes on the baking sheet, then transfer to a wire rack to cool completely.
Repeat the rolling and shaping process with the other batches of dough wile the first batch bakes.
Once the cookies are completely cool, dust them with powdered sugar. (The sugar will melt if you add it to hot or warm cookies.)