Augmenting Cash with my own functions

Unlike many other typescript libraries. Doing something like:

declare module 'cash-dom' {
    export interface Cash
        ensureClass(className, remove):Cash
    }
}

Doesn't work. Neither in the same .ts file, a seperate .d.ts file and with or without the export before the interface, in case that would.

Ofcourse, in the .d.ts file i'm adding

import 'cash-dom'
// repetition of previous code

How to augment Cash?