Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper or lightly grease it.
Make the base: In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy — about 2–3 minutes.
Add the wet ingredients: Mix in the egg and vanilla extract until smooth.
Stir in the dry ingredients: Gradually add flour, baking soda, salt, and cinnamon, mixing just until combined. The dough will be thick — that’s what you want!
Press the dough into the pan, smoothing it out evenly with a spatula or your hands.
Prepare the swirl: In a small bowl, mix together brown sugar, cinnamon, and melted butter. Drizzle and swirl it over the top of the dough using a butter knife. Don’t overdo it — gentle swirls will give you that beautiful marbled effect.
Bake for 22–26 minutes or until the edges are golden brown and a toothpick inserted in the center comes out clean. The center should still be soft — it’ll set more as it cools.
Cool completely in the pan on a wire rack before frosting.
Make the frosting: Beat together cream cheese and butter until smooth and creamy. Add powdered sugar and vanilla, and mix until fluffy and spreadable.
Spread the frosting generously over the cooled bars. Slice into squares or rectangles and enjoy!