1. Preheat the oven to 350°F (175°C).

2. Grease a baking dish with butter and set aside.

3. In a mixing bowl, beat eggs and brown sugar until light and fluffy.

4. Melt butter in a saucepan over low heat, then stir in cocoa powder until smooth. Add this mixture to the egg and sugar mixture.

5. Gradually mix in the milk, all-purpose flour, and baking soda until well combined.

6. Pour the batter into the prepared baking dish and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.

7. Let it cool before cutting into squares and serving.