Preheat oven to 350 degrees F. Grease two 6-inch round cake pans. Line with parchment paper. Set aside.
Whisk together flour, cinnamon, baking powder, baking soda, and salt. baking. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream butter. Add sugar and brown sugar and mix until combined. Scrape down bowl as needed to ensure thorough mixing.
Add eggs one at a time. Follow with the mashed bananas. Continue to mix until incorporated.
Alternate adding the flour mixing and milk. Start and end with the flour in three additions. Scrape down bowl as needed. Mix until just combined.
Divide batter evenly between the two prepared cake pans. Bake for 30-45 minutes until toothpick inserted in the center of cake comes out clean. Let cool in pan for a few minutes before unmolding and transferring to wire rack. Allow to cool to room temperature before slicing layers.
In the bowl of a stand mixer fitted with a paddle attachment cream together butter and tahini. Scrape down bowl as needed to ensure thorough mixing. Add confectioners' sugar, cinnamon, vanilla, and salt. Mix until buttercream is thick and smooth.*
Split each banana cake into two equal layers. You should end up with four layers total.
Place one layer of cake on cake board or serving plate. With an offset spatula slather a layer of tahini buttercream. Sprinkle chocolate chunks over buttercream. Top with second cake layer. Repeat adding buttercream and chocolate chunks. Continue to repeat until last layer of cake is on top.
Coat the top and sides of the cake with remaining tahini buttercream. Allow cake to chill in the fridge for 10-15 minutes to set up buttercream.
Remove from fridge and coat cake with cooled melted chocolate. Garnish the top of the cake with sesame seeds.
If consistency of buttercream is too thick, add 1 tablespoon of milk at a time until desired consistency is achieved. If buttercream is too thin, add confectioners' sugar 2 tablespoons at a time.