When rocks cool, crystals start to grow from multiple seeds because of thermal/chemical impurities. If the material is sufficiently homogenous in composition and temperature, then the crystals from different seeds will grow at the same rate. When they grow into each other, a boundary between different crystal domains will form. Crystal domains formed this way look like Voronoi cells.
The peanut butter cup looks like this because it's injected from several outlets with an approximately constant flow rate. Each injector outlet forms a Voronoi cell of peanut butter under it.
I know the former bit. I don't see how the latter obviously follows. You're basically saying it's related because it looks the same but it's clear from the picture it looks the same.
Imagine that the peanut butter is not poured from the top, rather coming from holes at the bottom of the cup. Initially there's a radially growing patch of peanut butter around each hole. When the peanut butter patches grow large enough, they touch, and form an approximately straight boundary (if the flow rate is equal for all outlets) like the boundary between crystal domains.
Of course, peanut butter is a fluid, so its dynamics are ultimately different from crystal growth, but at this scale, it produces a very similar phenomenon thanks to its high viscosity.
When rocks cool, crystals start to grow from multiple seeds because of thermal/chemical impurities. If the material is sufficiently homogenous in composition and temperature, then the crystals from different seeds will grow at the same rate. When they grow into each other, a boundary between different crystal domains will form. Crystal domains formed this way look like Voronoi cells.
The peanut butter cup looks like this because it's injected from several outlets with an approximately constant flow rate. Each injector outlet forms a Voronoi cell of peanut butter under it.