Melt butter in a large skillet over medium heat; cook and stir sunflower kernels and ramen noodles in hot butter until sunflower kernels are lightly browned and fragrant, 3 to 5 minutes. Remove from heat and transfer into a bowl to cool.

Stir sunflower seeds and ramen noodles with napa cabbage and spring onions in a large salad bowl. Whisk vinegar, vegetable oil, sugar, and soy sauce together in a separate bowl until sugar has dissolved; pour dressing over salad and toss.