Combine all ingredients in a small saucepan. Stirring frequently bring the mixture to a boil.
Remove from stove and serve warm.
Notes
While not traditionally used, I add a small amount of sugar to balance out the salty flavor of the soy sauce. Start with a small amount, taste, and adjust as necessary.
After combining your ingredients in a saucepan, bring them to a simmer over medium heat. You want the flavors to meld together, but boiling the sauce can result in an overly strong flavor.
Tempura dipping sauce is typically served at room temperature or slightly warm, not hot. This helps preserve the delicate flavor of the tempura.
BONUS TIP: This is optional, but grating some daikon radish into the sauce can add an extra layer of flavor and texture. You can also add some finely chopped green onions for additional flavor.