1. In a large saucepan set over high heat, add the kombu, soy sauce, sweet soy sauce, rice vinegar, mirin and granulated sugar. Add enough water to cover the kombu completely. Bring the liquid to a simmer over high heat, then reduce the heat to low and simmer gently until the kombu is very tender, 2 to 3 hours. Strain the mixture into a fine-mesh sieve set over a medium bowl, reserving the cooking liquid. Transfer the kombu to a blender and add ¼ cup of the cooking liquid. Blend until a thick, smooth purée forms, adding more cooking liquid as needed. Transfer the seaweed paste to an airtight container and cool completely before using. (Store in the refrigerator for up to 1 week.)

2. In a stand mixer fitted with the paddle attachment, add the butter, ¼ cup of the prepared seaweed paste (or 2 tablespoons of momoya) and the sea salt. Beat the mixture together until combined and lightly aerated, about 5 minutes. Serve the seaweed butter with the baguette slices, radishes and additional sea salt.

*Recipe Note: The seaweed paste can be substituted with 2 tablespoons of the prepared seaweed paste called momoya.