Step 1Using a vegetable peeler, peel the lemon in lengthwise strips, avoiding the white pith. Reserve the fruit for another use. Place the lemon peel in a sturdy bowl and muddle with the sugar cube until the peels release their oil and the sugar becomes moist and fragrant.
Step 2In a shaker without ice, shake the lemon peel, sugar, cassis and elderflower liqueur. Strain into a chilled punch bowl over a large block of ice that has been frozen with raspberries and blackberries in it. Add the Champagne and garnish with raspberries and blackberries.