Begin by making the garlic and herb butter. Place the garlic, parsley, 1/2 cup butter, 1/2 teaspoon salt and 1/2 teaspoon ground black pepper into a bowl and mix to combine.
Once combined, spoon the butter onto a plate lined with a sheet of parchment paper. Spread the butter into logs that are approximately 2 cm in width, 6 cm in length, and 1 cm in height. Once they are formed, place the plate in the freezer for 15 minutes.
While the butter is freezing, prepare the chicken. Use a sharp paring knife to remove the chicken tenders from the backside of the chicken breasts (if they are on). Set the chicken tenders aside until later.
Use the knife to make an incision into the thickest part of the chicken breast, large enough to fit the butter pieces. Make sure that you do not puncture through the chicken, or else all the butter will leak out once it cooks. Repeat for the remaining chicken breasts and set aside.
Add the remaining 1/2 cup of butter into a large pan over medium-high heat. Once the butter is melted, add the panko breadcrumbs, 1/2 teaspoon salt, and 1/2 teaspoon ground black pepper, and mix to combine. Gently toast the breadcrumbs for a minute until they absorb all the butter before transferring into a large bowl.
Add the flour to another large bowl, along with 1/2 teaspoon of salt and 1/2 teaspoon ground black pepper. Set aside.
Add the eggs to another large bowl along with 1/2 teaspoon salt and 1/2 teaspoon ground black pepper and beat to combine. Set aside.
Next, stuff the chicken. Remove the butter from the freezer. Gently push the butter into the incision in the chicken breast, making sure not to tear or puncture the meat. If the butter is too large, simply cut into smaller pieces and stuff the chicken breasts, leaving 2-3cm of space at the opening.
Take the leftover chicken tenders and stuff them into the incision, working to cover the opening and sealing in the butter. Again, if the chicken tender is too large, simply trim to size.
Once stuffed, tuck the thin end of the chicken breast underneath the stuffed area. This will ensure the chicken cooks evenly and give it a nice oval shape. Repeat for the remaining chicken breasts.
Preheat the oven to 375 degrees F. Line a large baking sheet with parchment paper and set aside.
Next, bread the chicken. Carefully dunk each chicken breast into the flour, followed by the egg, and then the breadcrumbs, making sure to fully coat the chicken at each step. Once breaded, place on the baking tray spaced at least 5 cm apart.
Bake the chicken breasts for 30 minutes. Once cooked, remove them from the oven and let cool for 5 minutes before serving.