Boil the noodles according to the package directions.
In a large skillet, heat the butter, miso, brown sugar, and soy sauce over medium heat for 2-3 minutes, stirring almost constantly, until butter is melted and sauce is slightly thickened.
Drain the noodles and add the noodles to the miso butter sauce. Stir to coat the noodles in the miso butter sauce.
Plate, and add toppings or mix-ins as desired.
Notes
When heating the miso butter sauce, lower the heat if the miso sauce seems like it is cooking too quickly or burning, and make sure to stir almost constantly while sauce is cooking.
Make sure to have any mix-ins/topping ready for adding right after the noodles are tossed in the sauce.
I like to pour any leftover sauce (if there is any left in the pan) over the noodles after plating.