Preheat oven to 350° F. Butter one 9 x 5 x 3-inch loaf pan. Whisk the flours, baking powder, baking soda, salt, and two tablespoons of the black sesame seeds in a medium bowl. Grind the remaining sesame seeds in a spice mill to form a thick paste, about 2 minutes. Set aside.
Using an electric mixer or a stand mixer fitted with a paddle attachment, cream the butter and sugar until well-combined, 2 to 3 minutes. Add the sesame paste and beat, occasionally scraping down the sides, until blended, 1 to 2 minutes. Add the egg and egg yolk and beat until pale and fluffy, 3 to 4 minutes. Beat in the mashed banana.
On low speed, beat in flour mixture in 3 additions, alternating with the buttermilk in 2 additions. (Begin and end with the dry ingredients.)
Spoon the batter into the prepared pan. Dollop peanut butter over top and use a knife to swirl it into the batter. Smooth the top, then sprinkle sesame seeds and granulated sugar over top. Bake for 45 minutes to 1 hour, until a tester comes out with only a few moist crumbs.
Cool completely in the pan before turning out and slicing.
A (former) student of English, a lover of raisins, a user of comma splices. My spirit animal is an eggplant. I'm probably the person who picked all of the cookie dough out of the cookie dough ice cream. For that, I'm sorry.