Great photo! This is a common difficulty that is even more of a problem when shooting outdoors.
The simplest solutions, as outlined above, would be using a reflector rather than a flash (this could work really well as you have a big window) or using a sheet to reduce the incoming light from the window and either something similar over your flash, or moving the flash further away (doubling the distance will reduce the power toe 1/4 of what it was - the inverse square law).
Alternatively, there are two direct ways you can get round this problem while still using the flash.
The first is to increase your sync speed. Although you're at the maximum sync speed for your camera, the Canon H430ex is capable of high speed synchronisation (HSS). This means the camera fires several pulses of light in order to synchronise at a higher shutter speed.
Note that reducing the shutter speed, however, will not reduce the exposure of the flash, which, as you've got the flash and ambient already well balanced, you need to do - as above you could either move the flash or put more between it and the subject.
(You may also read that using HSS reduces the power output of your flash - it does, but my understanding is that it reduces the maximum available output, i.e. not necessarily whatever power you happen to have selected. So as you're already using 1/64 this effect probably won't come into play.)
The problem with this method is that it won't work with normal wireless radio triggers. As a solution to this, you could either get a long TTL cable to use, or you could buy pocket wizard triggers.
The alternative would be to try using a neutral density filter. This will directly underexpose the whole image, and as you've already got the ambient-flash balance right this will work fine. There is a 1 2/3 stop difference between f2.2 and f1.2 according to (http://imaginatorium.org/stuff/stops.htm) so a 2-stop ND filter should do the job. Note that a 2stop filter is a 4x darkening factor. (Be wary of variable ND filters unless you're prepared to buy a really good one, they tend to be of poor quality)
Finally you could try achieving a shallower depth of field with a more telephoto lens, such as an 85mm or 70-200mm lens, however you would need to be further away from the subject which looks like it wouldn't be easy with this shot. Additionally this would result in less included in the background, so you probably wouldn't be able to see the edges of the bench in your picture. And if you really wanted to go for it you could get a medium format camera which would help achieve a shallower DOF, but that's probably the most unrealistic of all the options!