Why is making brownies an endothermic reaction?

Making brownies is not an endothermic reaction, it is an exothermic reaction.

An endothermic reaction is a chemical reaction that absorbs heat from the surroundings, causing the temperature of the surroundings to decrease. An exothermic reaction is a chemical reaction that releases heat into the surroundings, causing the temperature of the surroundings to increase.

When you bake brownies, the chemical reactions that take place release heat into the surroundings, causing the temperature of the oven and the brownie batter to increase. This is why brownies come out of the oven hot and need to cool down before they can be eaten.