Why Do Cookies Spread?

Big question: Why do cookies made with melted butter spread more?

Bake mini cookies with cold, soft and melted butter, then line them up to see how butter changes the spread.

What you need

• 6 tablespoons butter, in three 2-tablespoon pieces
• 6 tablespoons brown sugar
• 3/4 cup all-purpose flour
• 3 teaspoons milk
• Salt and baking soda
• 3 small bowls and a fork
• A small microwave-safe cup for melting butter
• A baking sheet lined with parchment paper (paper that stops sticking)
• A ruler
• A pencil

What to do

1. Get the butter readyThirty minutes before you start, take one butter piece out of the fridge so it can soften. When you are ready, wash your hands first and check every ingredient with your grown-up, who heats the oven to 350 F (175 C). Keep one butter piece cold. Your grown-up melts the third piece in the microwave-safe cup, lets it cool for 5 minutes and checks that it is warm, not hot, before it goes in your bowl.
2. Mix three doughsIn each bowl, mix one butter piece with 2 tablespoons brown sugar, 1/4 cup flour, 1 teaspoon milk and a pinch each of salt and baking soda. Mash the cold butter in with the fork. No tasting the raw dough.
3. Shape and labelScoop each dough into two ping-pong-size balls. Put each kind in its own row and write cold, soft or melted on the parchment.
4. Bake and watchYour grown-up bakes them in the hot oven for 10 to 12 minutes. Watch through the oven window from a step back, and do not touch the door, because it gets hot. Which one spreads first?
5. MeasureOnce cool, measure across each cookie. Break them open: thick or thin, chewy or crisp?

What is happening?

Butter is mostly fat. Cold butter is solid, so it melts slowly in the oven and the edges set before the dough can flow outward. Melted butter is already liquid, so the dough spreads before the heat firms it up, and soft butter lands in between. That is why bakers chill dough for thick cookies.

The oven, the microwave, the hot baking sheet and hot melted butter are grown-up jobs. No tasting the raw dough, because raw flour can make you sick.

Try it

• 
  Junior Bakers

  Chocolate Chip Cookie

• 
  Junior Bakers

  Vanilla Brown Sugar Cookies

Check yourself