Meanwhile, melt 1 Tbsp. butter in a small saucepan over medium heat. Add miso and cook, stirring and scraping bottom of pan constantly, until miso darkens a few shades and smells very toasty and nutty (it will stick to pan), about 4 minutes. Pour in 1⁄4 cup water and whisk until incorporated, scraping bottom of pan to release any browned bits. Add sriracha and remaining 4 Tbsp. butter; cook, whisking constantly, until butter is melted and sauce is smooth. Season with salt. If your miso is a bit lumpy, use an immersion blender to blend sauce directly in pan until smooth (or, carefully blend in a regular blender).