Stir together the sour cream and minced chipotle in a small bowl. Cover and refrigerate for at least 30 minutes.
Cut tomatoes into 1/4-inch-thick slices. Whisk together flour, salt, and chili powder in pie plate. Pour buttermilk into the second pie plate. Pour panko into the third pie plate.
Heat oil in the large cast iron skillet over medium heat. Meanwhile, dredge tomato slices in the flour mixture and shake off the excess. Dip into the buttermilk on both sides, then immediately dredge slices in panko.
Cook tomato slices in hot oil until brown and crispy, about 3 to 5 minutes per side. Drain thoroughly on the paper towels. Serve immediately with the chipotle sour cream. Best eaten within an hour.
If you don’t have buttermilk, you can make your own. Combine 2 tablespoons fresh lemon juice plus enough lowfat milk to equal 1 cup. Let it sit at room temperature for about 15 minutes, then it’s ready to use.