Bring a saucepan of lightly salted water to a boil.

Add soba noodles and boil until al dente (about 2 minutes).

Drain noodles and immediately plunge into ice water to stop cooking.

Drain noodles again and set aside.

Trim and thinly slice scallion greens; set aside.

Halve the white scallion base lengthwise and set aside.

Remove orange zest, avoiding the white pith.

Reserve the orange for later use.

Rub orange zest and ginger slices with salt.

Create a 'flavor mat' in a bamboo steamer with scallion bases, ginger, and orange zest.

Layer half of the basil over the flavor mat.

Arrange shrimp on top, then add remaining ginger, orange zest, and basil.

Distribute red pepper strips around the shrimp.

Top with sliced shiitake mushrooms.

Pour water into the steamer pan and bring to a simmer.

Add dashi powder and stir until dissolved.

Place the steamer basket over the simmering base, cover, and steam until shrimp turn pink and are cooked through (about 6 minutes).

Remove steamer and keep covered to keep warm.

Add noodles to the simmering broth to heat through.

Divide noodles and broth among warmed bowls.

Carefully transfer red pepper and mushroom slices to the bowls.

Arrange 4 shrimp per bowl, overlapping them over the noodles.

Discard the flavor mat.

Julienne 10 basil leaves.

Sprinkle julienned basil and scallion greens over each bowl.

Garnish with a fresh basil sprig and serve.