I thought "focussing" was the British way of spelling (English is not my mother tongue). But then I ran into this blog post, so now I think I prefer "focusing" after all:
"The spelling rule is that if the first syllable of two is the one that is stressed, don't double the final consonant, but this seems to be overridden by the general notion of doubling the final consonant before adding -ed and -ing. People often can't see the point. If begin becomes beginning why shouldn't focus become focussing? The subtlety of the stress being on the second syllable in the case of begin and on the first syllable in the case of focus escapes them."
First of all, I landed here by looking up the proper spelling of the word. But the two-S spelling is clearly for suggestive of the right pronunciation, so to me it is more logical.