Cut the softened 2 cups (16 oz / 455g) of butter into 1-inch (2. 5 cm) pieces and place them directly into a food processor fitted with a standard metal blade.
Add the 0. 33 cups (0. 8 oz / 24g) chopped fresh basil, 1 tbsp. (0. 25 oz / 7g) freshly ground nutmeg, 1 tbsp. (0. 1 oz / 3g) chopped fresh coriander, and 1 tsp. (0. 2 oz / 6g) salt to the food processor bowl.
Cover the processor and pulse the mixture until the fresh basil and coriander leaves are finely chopped and uniformly blended into the butter.
Remove the blade and manually stir in the 1 tbsp. (0. 3 oz / 9g) black poppy seeds if you are using them for an extra layer of flavor and texture.
Transfer your finished Basil-Nutmeg Butter into decorative molds for entertaining, or wrap it tightly in wax paper.
When well-sealed, the butter can be kept frozen for up to 3 months.