Fix an equilateral triangle group T i = a, b; a i , b i , (ab) i with i ≥ 6 arbitrary. Our main result is: for every presentation P of every countable group Q there exists an HNN-extension T P of T i such that Out(T P ) ∼ = Q. We construct the HNN-extensions explicitly, and examples are given. The class of groups constructed have nice categorical and residual properties. In order to prove our main result we give a method for recognising malnormal subgroups of small cancellation groups, and we introduce the concept of "malcharacteristic" subgroups.