When the summer heatwave rolls through Chicago, all most of us can think about is staying indoors with the AC cranked up. This year has been no exception. The heat index on Wednesday of this week was expected to reach 115F! It can also be torturous to bake in your kitchen on days like that. So rather than cranking up the oven to make chocolate chip cookies, I decided to use DH’s car as a solar oven.

But really, any car will do. Your Prius works just as well as an eco-terrorizing Hummer. The key here is to have 3+ hours of 95F as your external temperature. And I suppose non-tinted windows would help your car reach the heat necessary to actually bake cookies.

It was touch and go there at the beginning because some clouds moved over the sun. That brought the temperature inside the car down to 130F. You need the temperature inside the car to be at 180F or higher.

What you need:

  • Sunny day with temperature of 95F or higher for at least 3 hours.
  • Car with windows that close up tight.
  • Place to park the car so sun reaches the windshield for the duration of baking time.
  • Baking sheets covered in aluminum foil.
  • Towels.
  • Potholders.
  • Eggless and dairy-free dough.
  • Thermometer.


  1. Check the hourly forecast to ensure you’re going to have at least 3 hours of 95F + heat.
  2. Park your car so the windshield is facing the sun for the duration of baking. Take nearby trees into consideration. Close it up to pre-heat while preparing the dough.
  3. Pick a recipe that does not contain eggs or milk. The car won’t get hot enough to make non-vegan dough safe to eat. There are a lot of vegan cookie recipes you can search for via Google. The recipe I used was Garrick’s Chocolate Chip cookies on page 215 of Kelly Peloza’s cookbook, The Vegan Cookie Connoisseur.
  4. Foil your baking sheets with the shiny side up to reflect more heat.
  5. Cover your dashboard with towels in case something melts off the baking sheets.
  6. Position an oven thermometer on the dash so you can monitor the temperature by looking through the window. You’re shooting for 3 hours of 180F inside the car. Mine only reached 160F so I think the Yaris’ windshield may have some sort of coating on it to cut down on heat. So I left my batch of cookies in an hour longer than normal.
  7. Place the cookie sheet, loaded with dough, on the dash. Close the doors and start timing. At 1 hour, the cookies will be flattening. At 2 hours, they’ll look very flat and smooth. At 3 hours, you can pretty much tell through the windshield if the cookies are close to done. Chocolate chips will start poking through the dough, deforming the smooth surface. You can open the door and test a cookie by pressing the center gently with your thumb. If it’s firm, and a spatula lifts them easily, they’re done.
  8. Using potholders, remove the trays to your kitchen, setting them on trivets to cool. I kid you not, these trays will burn your hands. Use the same precaution you’d use removing trays from your kitchen oven.


  • free energy
  • eco-friendly
  • easy
  • car smells wonderful
  • conversation starter among co-workers and neighbors
  • crispy edges and chewy centers
  • no way to burn the cookies


  • not a totally crunchy cookie but great for softer cookies
  • not hot enough to caramelize sugars in cookies
  • takes a while
So no more belly aching about the weather, eh? Let’s get baking!