Preheat oven to 180°C (350°F) and grease a cake pan.
In a bowl, whisk yogurt, honey, eggs, and milk until smooth.
Add whole wheat flour, cocoa powder, and baking soda.
Mix until just combined without overmixing.
Pour batter into prepared cake pan.
Bake for 30–35 minutes until a toothpick comes out clean.
Let cool completely before slicing and serving.