Buttered Steak Bites with Mushrooms

Bahan-bahan (4 porsi)

• 1 1/2 pounds skirt steak
• 1 teaspoon kosher salt
• 1/2 teaspoon freshly ground black pepper
• 4 tablespoons butter (divided)
• 6 ounces small crimini or button mushrooms (sliced in half)