Fill a large wine glass with ice.

Pour the elderflower liqueur and homemade lavender syrup over the ice, stirring gently to combine.

Top the mixture with the Prosecco and club soda.

Stir once from the bottom up to mix the layers without losing the carbonation, then garnish with a lemon wheel and a sprig of fresh lavender.

Combine the water, sugar, lavender buds, and butterfly pea flowers, if using, in a small saucepan.

Bring the mixture to a gentle boil over medium heat, stirring until the sugar is fully dissolved.

Remove the saucepan from the heat, cover it with a lid, and let the mixture steep for 15 to 20 minutes to infuse the flavors and color.

Pour the syrup through a fine-mesh strainer into a clean jar to remove the botanicals, then let it cool completely before use.