You will need to present the voucher.
Though it doesn't say as much on the voucher, I doubt they'll honour the 2-for-1 hot chocolate deal at the same time.
These tiny cake slices usually cost about $11. The cheapest dine-in hot drink is probably $3.80 (short black/macchiato).
Heyya,
have you confirmed they are accepted in Australia?
webpage says it's from France? (tld .fr)
cheers
Michael