Prep the pan and oven. Heat the oven to 325°F (163°C). Line a 9x9-inch baking pan with parchment, leaving overhang on two sides for easy lifting. Lightly grease the parchment.

Make the crust. Mix cookie crumbs, melted butter, sugar, and salt until the texture resembles wet sand. Press firmly and evenly into the pan. Use the bottom of a measuring cup to compact. Bake for 8–10 minutes, then let cool slightly.

Beat the cream cheese base. In a large bowl, beat cream cheese and sugar on medium speed until smooth and fluffy, about 2–3 minutes. Scrape the bowl. Add eggs one at a time, mixing on low until just combined.

Add the rest. Mix in sour cream, vanilla, cornstarch, and salt on low speed until smooth. Avoid overmixing to limit air bubbles.

Color a few spoonfuls. Transfer 1/2 cup of the batter to a small bowl. Divide into 2–3 ramekins and tint each with a different gel color. Add a splash of cream to each to keep it fluid.

Assemble. Pour the plain batter over the crust and smooth the top. Dollop small spoonfuls of the colored batter across the surface. Use a toothpick or skewer to gently swirl. Don’t overdo it—two or three passes is enough.

Bake. Bake at 325°F (163°C) for 28–34 minutes. The edges should be set and the center slightly wobbly. Avoid browning on top.

Cool gradually. Turn off the oven, crack the door, and let the bars sit for 10 minutes. Move the pan to a rack and cool to room temperature.

Chill. Cover and refrigerate at least 4 hours, preferably overnight, to fully set.

Finish and serve. Lift out using the parchment overhang. Add sprinkles, candy eyes, or crushed candies if you like. Cut into bars with a warm, dry knife for clean slices.