Preheat oven to 350°F (175°C) and grease a 10-12 cup bundt pan. In a large bowl, beat together the cake mix, pudding mix, eggs, oil, water, mashed bananas, and sour cream until smooth, about 2 minutes. Pour batter into the prepared pan.

Bake for 55-65 minutes, until a skewer inserted near the center comes out with moist crumbs. Cool in the pan on a wire rack for 10 minutes, then invert onto the rack and let the cake cool until warm.

Place vanilla wafers in a zip-top bag and crush into fine crumbs with a few small pieces. Stir in the melted butter and brown sugar until the mixture forms clumps.

While the cake is still warm, gently press the crumble mixture over the top in an even layer. Let it set for 15 minutes before slicing.