  We associate to a bimonoidal functor, i.e. a bifunctor which is monoidal in
each variable, a nonabelian version of a biextension. We show that such a
biextension satisfies additional triviality conditions which make it a bilinear
analog of the kind of spans known as butterflies and, conversely, these data
determine a bimonoidal functor. We extend this result to $n$-variables, and
prove that, in a manner analogous to that of butterflies, these
multi-extensions can be composed. This is phrased in terms of a multilinear
functor calculus in a bicategory. As an application, we study a bimonoidal
category or stack, treating the multiplicative structure as a bimonoidal
functor with respect to the additive one. In the context of the multilinear
functor calculus, we view the bimonoidal structure as an instance of the
general notion of pseudo-monoid. We show that when the structure is ring-like,
i.e. the pseudo-monoid is a stack whose fibers are categorical rings, we can
recover the classification by the third Mac Lane cohomology of a ring with
values in a bimodule.
