Shrimp and Beef Skewers with Soy and Scallion Butter

Name: Shrimp and Beef Skewers with Soy and Scallion Butter
Ingredients: 2 sticks unsalted butter, at room temperature; 5 scallions, pale green and white parts only, finely chopped; 3 cloves garlic, minced; 3 tablespoons soy sauce; 1/3 cup extra-virgin olive oil; 4 cloves garlic, minced; 1 teaspoon kosher salt; 1 teaspoon freshly ground black pepper; 8 ounces (about 12) medium-size button or cremini mushrooms, halved; 1 pound (about 28) large shrimp, peeled and deveined; 1 pound sirloin steak, about 3/4-inch thick, cut into 28 (1-inch) pieces; 20 (6 to 8-inch) wooden skewers, soaked in water for 30 minutes to prevent scorching