Preheat the oven to 350ºF. Generously grease the bundt pan with butter.

Cream the softened butter and brown sugar together until light and fluffy, about 3-5 minutes.

Gradually add the beaten eggs into the butter and sugar mixture. Make sure that the eggs are completely incorporated after each addition.

In a small bowl, add the mashed banana, Greek yogurt, and vanilla. Fully combine.

In another bowl, sift the flour, protein powder, and baking soda together. Fully combine.

Gently fold in one third of the banana mixture, followed by one third of the flour mixture. Continue adding the rest until fully combined.

Remove one third of the banana cake batter and place in a small bowl. Add the cocoa powder and whole milk. Fully combine.

Add the dark chocolate chips to the remaining banana cake batter and gently fold in until the chocolate chips are evenly dispersed in the batter.

Alternatively add the chocolate and banana cake batters into the greased bundt pan, swirling the batter to create a marbled design.

Bake for 45-50 minutes until the cake reaches 210ºF or a toothpick comes out clean.

Remove the cake from the oven and cool for 15 minutes before flipping onto a cooling rack. Let the cake to cook completely before serving.