How do I add melted chocolate to cake batter without it seizing?

I made this cake yesterday and it was just a nightmare to make. Every time I got to the step where you take 1T of cake batter and quickly mix it into the melted chocolate it just seized up immediately. I did this 4 times, and it failed every time. I ended up adding a fairly sizable amount of boiling water to it to thin it back out, but probably my cake texture is wrong now. I haven’t tried it yet.. the birthday is tomorrow.

So how do I do this without it crumbling up? What am I doing wrong!!

What I did: Melted chocolate in microwave (slowly) and was sure it was not overcooked. Then, added a tablespoon of cake batter to it which caused it to crumble up. This step, according to the recipe, was supposed to prevent that from happening..