Preheat your Traeger Grill to 350 degrees F. Spray three 8-inch or four 6-inch pans with non-stick spray, line the bottoms with parchment paper and spray again. Set aside.
In a medium sized saucepan, brown the butter over medium heat on your stove. You want the butter to boil and start to pop. At this point, the butter will also start to cook. Keep a close eye on it and let it cook until it's browned and has some dark specs. Remove from heat and let cool back to room temperature. I suggest putting the butter in a bowl and refrigerating it so it can harden slightly, or making the browned butter a day ahead.
In the bowl of a stand mixer fitted with a paddle attachment, combine the cooled butter, oil, sugar and salt. Beat on medium speed for about 2 minutes, until light and fluffy.
Turn the mixer to medium-low and add the eggs and egg yolks, one at a time, scraping down the sides of the bowl in between each addition.
With the mixer on low, alternately add the flour and baking powder with the buttermilk, starting and ending with the flour. Mix until combined.
Add the butter extract and vanilla extract. Mix on low until combined.
Evenly distribute the cake batter in the three pans, about 16 to 17 ounces of batter in each 8-inch pan or 12 ounces in each 6-inch pan.
Bake the cakes on the Traeger for about 23 to 25 minutes, until a toothpick inserted in the center of the cakes comes out with only just a few crumbs on it. At this point, you'll also want to put the peaches on the Traeger as well. The peaches will cook for about 40 to 45 minutes.
Cool the cakes on wire racks for about 10 minutes. Carefully invert the cakes onto cooking racks to cool completely. If you don't plan to stack and decorate the cake right away, level the cakes if needed and then immediately wrap each layer in plastic wrap or tin foil and freeze.
About an hour before assembly, remove the cake layers from the freezer.
Let the peaches cool completely before using in the cake.