To make the tayberry syrup:
Combine sugar and 1 cup water in a saucepan, and cook over low heat until dissolved. Add tayberries, raise heat, and simmer for 5 minutes. Strain through a fine mesh sieve and chill.
To make the cocktail:
In a cocktail shaker, dry shake all ingredients but the garnish for 5 seconds. (“Dry shake” is bartender-speak for shaking a drink without ice.) Add ice to shaker and shake vigorously for 20 seconds. Strain into a chilled Nick and Nora glass. Garnish with magnolia flower and serve.