I started off with the filled chocolates. As per the suggestion of klik, I first filled the forms with a bit of chocolate, let that cool off and then carved a small hollow in the chocolate. Then I filled it with the mixture and poured chocolate on top of that -- and let it cool off completely. They came out quite OK!
Next, I melted another batch of chocolate. I took some instant coffee and added some water to make it a smooth mixture. But when I mixed the coffee into the chocolate, the water reacted quite badly with the chocolate and the whole batch became lumpy and un-usable. I threw it out.
There has to be a better way to do this.