Preheat the oven to 350°F. Spread the walnuts in a single layer on a baking sheet. Toast for 6-8 minutes, then cool completely.
Sift together the both flours, pudding mix, baking powder, baking soda and salt. Set aside.
In the bowl of a stand mixer, cream together the softened butter and sugar, until pale yellow and fluffy. Around 3 minutes. Add the eggs one at a time beating well after each addition. Stop the mixer and scrape the sides of the bowl as needed.
I a separate bowl, whisk the vanilla into the buttermilk. Add the sifted dry ingredients to the creamed mixture alternately with the buttermilk.
After all of the ingredients have been added, increase the mixer speed, and beat for 2 minutes until fully combined. Stop to scrape the side of the bowl and the beater as needed.
By hand, mix in the chocolate chips, 1 1/2 cup toasted walnuts and cubed banana into the batter.
Spray a standard bundt pan with baking spray, then spread the thick batter into the pan. Bounce the pan on the counter a few times to remove the air bubbles.
Bake for 45-55 minutes, or until a tester inserted into the center comes back clean.
Cover the top with aluminum foil to prevent over-browning, if needed. Cool completely in the pan, then invert onto a cake plate.
To make the Milk Chocolate Ganache:
Melt the chocolate chips and heavy cream in a microwave safe dish. Heat in 20 second increments stopping to stir each time. Repeat until completely smooth.
Drizzle over the cooled cake and sprinkle the top with toasted walnuts.