If brining the turkey, start the night before. Add water, brown sugar and salt to a large pot that will hold the entire turkey breast. Stir to combine. Add the turkey. Place a lid on top and chill in the fridge overnight.
One hour before you are ready to cook, take the turkey out of the brine and pat dry with paper towels. It's best if the turkey comes to room temperature before you bake and spread the miso butter under the skin. If you have a cold turkey breast, the butter will solidify and be difficult to spread. Preheat the oven to 375 degrees. Set the turkey aside to dry more while you prepare the miso butter and glaze.
To a medium bowl, add the softened butter, miso paste, garlic, 2 teaspoons brown sugar, ginger, orange zest and 1 teaspoon salt. Mix until combined.
If your turkey still has the back bone and ribs, you can remove them to speed up the cooking time. Simply flip the turkey over and use very strong kitchen shears to cut the back bone and ribs out of the chicken breast.
Place the turkey breast in a small roasting pan or 15-inch cast iron skillet. Use your fingers to separate the skin of the turkey. Set 1/4 of the miso butter aside. Using your fingers or a spoon, gently spoon the remaining 3/4 of the miso butter under the skin. Spreading it as far as it will go. Sprinkle the remaining 1 1/2 teaspoons salt all over the top of the turkey breast.
Place in the oven and bake for 1 hour. While the turkey bakes, make the glaze. Take the rest of the miso butter and microwave in 15 second increments until it has melted completely. Whisk the ingredients together. Add in the soy sauce and orange juice. Whisk to combine. After one hour of cooking, open the oven, slide the turkey out and brush with the glaze. Pour 1/2 cup of chicken stock in the bottom of the pan and use a wooden spoon to scrape up any bits that are sticking to the bottom. Place the turkey back in the oven. Continue to bake until the turkey reaches and internal temperature of 165 degrees with a meat thermometer. If at any point, the turkey starts to get too caramelized and brown, tent with foil. Additionally if the chicken broth at the bottom of the pan starts to dry up and burn, pour 1/4 cup more into the pan to avoid a burnt bottom.
Let the turkey rest 15 minutes before carving. To carve, take your knife to the top of the breast bone and slice all along the breast bone removing the entire breast. Do the same on the other side. Then slice each breast into 1/4-inch or desired thickness slices. Serve with the pan juices.