For an arbitrary set B of Boolean functions satisfying a certain condition, we give a general method of constructing a class C_B of read-once Boolean formulas over the basis B that has the following property: For any F in C_B, F can be transformed to an optimal formula (i.e., a simplest formula over the standard basis {AND, OR, NOT}) by replacing each occurrence of a basis function h ∈ B in F with an optimal formula for h. For a particular set of basis functions B^* = {AND, OR, NOT, XOR, MUX}, we give a canonical form representation for C_B^* so that the set of canonical form formulas consists of only NPN-representatives in C_B^*.