Im just about to make a brownie for a friend tonight, i thought i had everything i needed to make it and i dont, and i cant get to the shops!!

I need 100g dark choc and 100g milk choc to melt to make brownie mix but i only have Dark choc. I f i just use 200 g of that ... which i have...what else should i do i keep the taste the same? more sugar? some milk? nothing? dont want it to be too rich or bitter??

any clues??