Adjusting the process for flavors

I created my first batch using 70% cacao, 28% sugar, 1.5 Vanilla & .5 Lecithin

The result is good for my first attempt, however tit builds up to a metallic taste.

I want to try to adjust that out. Any suggestions? I was thinking I should roast longer. I roasted for 16 minutes 2.5lbs. I stopped once I started hearing the popping.