In a small bowl, mix together the softened butter, parmesan and garlic powder until smooth.
Spread the mixture the outside of each slice of bread. On the inside of the first piece of bread, sprinkle the shredded gouda and white cheddar (or place the slices) evenly across and top with the second piece of bread (butter side out).
Place in a skillet over medium-low heat. Cover and cook for 2-3 minutes until the cheese is starting to melt and the bottom of the bread is lightly toasted.
Carefully flip the sandwich over and cook for an additional 1-2 minutes until the bottom is golden.
Slice and serve!
Notes
LeftoversI recommend enjoying it fresh. But if needed, allow to cool completely on a wire rack so that it doesn't get soggy and then store in an airtight container in the refrigerator for up to 3 days. Reheat your sourdough grilled cheese in a frying pan over medium-low heat for 4-5 minutes, flipping halfway through until warmed.