Combine 1 cup dried hibiscus, 1 cup sugar, and 1 cup water in a small saucepan. Bring to a gentle boil over medium heat, stirring until sugar dissolves. Reduce heat and simmer 5 minutes until the liquid is deep magenta. Remove from heat, let steep 15 minutes. Strain through a fine mesh sieve, pressing solids to extract liquid. Cool to room temperature; refrigerate in a sealed container for up to 2 weeks. Yield about 1 cup.

Assemble cocktail

Chill glass

If desired, place a highball or Collins glass in the freezer for 5 minutes or fill it with ice water while you prepare the ingredients to chill it.

Combine base ingredients

In a cocktail shaker, add 2 oz Hendrick's gin, 0.5 oz raspberry liqueur (or 0.5 oz fresh raspberry purée), 0.5 oz fresh lemon juice, and 0.5 oz hibiscus syrup. Fill the shaker halfway with ice.

Shake

Shake vigorously for 10–12 seconds until the shaker is well chilled and slightly frosted.

Strain over ice

Discard ice or water from the chilled glass (if used). Fill the glass with fresh cubed ice (about 1 cup). Double-strain the shaker contents into the glass to remove any raspberry seeds or ice shards.

Top with grapefruit soda

Gently top the cocktail with 3 oz chilled grapefruit soda. Stir once gently with a bar spoon to combine — do not over-stir to preserve carbonation and layered color.

Garnish and serve

Thread 3 fresh raspberries on a cocktail pick or float them on top. Add a thin grapefruit wheel or a twist on the rim and optionally a mint sprig for aroma. Serve immediately with a straw or sip directly.