Both sentences are possible, depending on what you want to convey.
We use the simple past (I wrote) to talk about things that happened in the past. You are thinking at that moment of the last time you wrote it.
We use the present perfect (I've written) to talk about finished events that are connected with the present. So if you use the present perfect tense, since I have written, you are thinking of the gap in time since you last did.
Hope this helps.