Preheat the oven to 175°C (350°F). Grease and line an 8-inch (20 cm) cake pan.

In a large bowl, mash the bananas.

Add the Greek yogurt, eggs, oil, brown sugar, and vanilla. Whisk until smooth.

In another bowl, combine the flour, cocoa powder, baking powder, baking soda, and salt.

Gradually fold the dry ingredients into the wet mixture until just combined.

Fold in the chocolate chips, if using.

Pour the batter into the prepared pan and smooth the top.

Bake for 35–40 minutes, or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 10 minutes before transferring it to a wire rack