Najisha, over-mixing is the #1 cause of tunneling in cakes. This is because too much gluten forms in the batter, which makes it harder for the gasses inside to get out during baking. They build up pressure until they push all the way out like a volcano, leaving you with tunnels. The best solution is to just barely mix your flour and wet ingredients together enough to get everything wet, and then leave it alone. Some lumpiness is just fine! Kat@KAF

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