Beat softened butter, cream cheese, and granulated sugar with electric mixer on medium speed for 2-3 minutes until light and fluffy.
Add egg and vanilla extract, beat until fully combined.
In separate bowl, whisk together flour, baking powder, and salt.
Gradually add dry ingredients to wet ingredients, mixing on low speed until just combined - don't overmix.
In small bowl, mix brown sugar, cinnamon, and softened butter into spreadable paste.
Set cinnamon filling aside while you roll the dough.
Roll, fill, and chill the dough
Place dough between two sheets of parchment paper.
Roll into rectangle about 12x8 inches and ¼ inch thick.
Remove top parchment sheet and spread cinnamon filling evenly over dough, leaving ½ inch border on all sides.
Starting from one long side, roll up dough tightly using bottom parchment to help lift and roll.
Pinch seam closed and roll gently to even out the log.
Wrap rolled dough log tightly in plastic wrap.
Refrigerate for at least 2 hours or overnight - this is NON-NEGOTIABLE for defined swirls.
Slice and bake your cookies
Preheat oven to 350°F and line baking sheets with parchment paper.
Remove chilled dough log from refrigerator and unwrap.
Use unflavored dental floss or very sharp knife to slice dough into ¼-inch thick rounds.
For dental floss method: slide floss under log, cross ends over top, and pull tight to slice cleanly.
Place cookies 2 inches apart on prepared baking sheets.
Bake for 12-14 minutes until edges just start to turn lightly golden.
Don't overbake - cookies will firm up as they cool.
Let cool on baking sheet for 5 minutes, then transfer to wire rack.
Make glaze and finish cookies
Beat softened cream cheese in medium bowl until smooth and creamy.
Add powdered sugar, 2 tablespoons milk, and vanilla extract.
Mix until smooth and drizzle-able - add more milk if too thick.
Drizzle cream cheese glaze over slightly warm cookies using spoon or piping bag.
Let glaze set for 10-15 minutes before serving or storing.
Cookies should show visible cinnamon swirls and have soft, tender texture.