In a large pot, melt the butter with the corn syrup, brown sugar and cocoa.
Increase temperature to medium, stirring a lot, until mixture starts to boil.
Let it boil for 2-3 minutes.
Remove from heat and add vanilla.
Stir.
Add buckwheat puffs and stir to combine.
Put mixture into a greased 9 x 13 pan, and lightly press down.