covariance cupcakes

My stats class had their lab test today, and last week I thought of a way to combine two of my greatest loves - geeking out & baking - into one sweet treat for my students. Behold:

Their extra credit was to correctly identify the formula represented in this cupcake mosaic.

Can anyone identify the formula on the middle cupcake?

I'm a decent baker and geek, but I think I'll leave the cake decorating to my sister.