Preheat oven to 350 degrees F. Grease and flour a 9x13 inch cake pan.

In a large bowl or stand mixer, beat together the yellow cake mix, Peach Jell-O powder, eggs, and oil until well combined.

Cut the frozen peach slices in half. Gently fold the peach pieces into the cake batter using a sturdy wooden spoon.

Pour the batter evenly into the prepared cake pan. Bake for 40-45 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached (very moist crumbs are okay). The edges will be quite brown.

Let the cake cool completely on a wire rack before frosting.

While the cake cools, prepare the frosting. In a 2-quart saucepan, combine the butter, brown sugar, and heavy cream. Stir over medium-high heat until it reaches a rolling boil.

Boil for exactly 1 minute, stirring frequently. Remove from heat and stir in the vanilla extract.

Prepare an ice bath. Carefully place the saucepan with the frosting into the ice bath (ensure no water gets into the frosting). Let the frosting cool, scraping the sides occasionally, until it reaches room temperature and has thickened considerably.

Using a hand mixer, beat the cooled frosting for 3-5 minutes until it is thicker but still pourable. Spread the frosting evenly over the completely cooled cake.