1. Prepare Oven and Pan:

• Preheat the oven to 350°F (175°C).
• Grease a 9×13 inch cake pan well.

2. Mix the Cake Batter:

• In a large bowl or the bowl of a stand mixer, beat together the yellow cake mix, the dry Peach Jell-O powder (both boxes), the 4 eggs, and the 1 cup of oil until well combined (follow mixer time on cake box if preferred, usually about 2 minutes).

3. Prepare and Add Peaches:

• Open the bag of frozen sliced peaches. Working quickly, cut each frozen slice in half or into bite-sized pieces.
• Using a sturdy wooden spoon or spatula, gently fold the frozen peach pieces into the cake batter. The batter will be thick, and the peaches should remain frozen.

4. Bake the Cake:

• Pour the batter (with peaches) into the greased 9×13 inch cake pan and spread it as evenly as possible.
• Bake at 350°F (175°C) for about 40-45 minutes.
• Check for doneness by inserting a toothpick into the center; it should come out clean or with very moist crumbs attached (not wet batter). Note that the edges of the cake will likely be quite brown – this is normal for this cake.

5. Cool the Cake:

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

6. Make the Brown Sugar Frosting:

• While the cake cools, make the frosting. In a 2-quart saucepan, combine the salted butter, packed brown sugar, and heavy cream.
• Stir together over medium-high heat until the mixture reaches a full boil (bubbles actively all the way across the top).
• Once boiling, set a timer for exactly 1 minute and continue to stir frequently while it boils.
• Immediately remove the pot from the heat. Stir in the vanilla extract.

7. Cool the Frosting (Ice Bath):

• Prepare an ice bath: Fill a large bowl (larger than your saucepan) partway with ice and cold water.
• Carefully set the hot saucepan containing the frosting into the ice bath. Be careful not to get any water into the frosting itself.
• Let the frosting cool in the ice bath, scraping the sides of the pot down occasionally with a spatula, until the frosting reaches room temperature and becomes much thicker (consistency will be very thick, almost solid). This takes time and patience.

8. Beat the Frosting:

• Once cooled and thickened, remove the pot from the ice bath.
• Use a hand mixer to beat the frosting directly in the pot (or transfer to a mixing bowl if easier) for about 3-5 minutes. It will transform from very thick to a lighter, thicker-but-still-pourable/spreadable consistency.

9. Frost the Cake:

• Ensure the cake is completely cool.
• Pour the beaten brown sugar frosting over the cooled cake.
• Use an offset spatula or the back of a spoon to spread the frosting evenly over the cake. Work somewhat quickly as it can start to set.

10. Store:

• The cake keeps well on the counter, covered, for a day or two. After that, store it covered in the refrigerator