Put some Martini glasses (or champagne flutes) into the freezer. Mix the clementine juice, vodka and Cointreau in a jug, then chill for 1 hr (or up to 1 day). Thinly slice the 2 clementines to use as a garnish.

To serve, put a clementine slice into each frosty glass. Fill almost halfway with the chilled jug mixture, then open the bottle of fizz and top up the glasses.