Recipe-development draft. This recipe has not been kitchen-tested. Confirm the yield, bake time, texture and storage quality before publishing.
These are the bars for a little celebration that does not need a whole cake. Vanilla and white chocolate make the base, with rainbow sprinkles folded through the batter and scattered lightly over the top.
One pan, no frosting
Use an 8-inch square metal pan and line it with parchment handles. The finished slab lifts out for slicing, so you can make tidy squares without trying to cut around the corners of the pan.
Choose the right sprinkles
The formula specifies bake-stable jimmies. Keep the tiny nonpareils and large hard decorations for another use until their behavior has been tested here. Measure 45 g in total, then divide that into 30 g for the batter and 15 g for the surface.
Mix gently after the flour goes in
The butter and sugars can be whisked, but switch to a spatula once you add the flour. Fold in the white chocolate and sprinkles at the end. The batter is intended to be thick enough to spread into the corners rather than pour like cake batter.
How to check the middle
Look for set, lightly golden edges and a center that no longer appears liquid. If a toothpick hits melted white chocolate, move to another spot. A chewy bar still needs a baked center; wet batter is not the goal.
The cooling time is part of the recipe
Allow about 90 minutes before slicing. Cutting warm bars makes it difficult to tell whether the center has set and tends to drag the white chocolate through the crumb. Give the pan its time on the rack, then use the parchment to lift out the slab.
For a specific occasion, the sprinkle colors can change while the type and total weight stay the same. Keep a note of the brand used during testing so the final recipe and photographs match.