Preheat oven to 350°F (180°C). Grease and flour a 13x9-inch baking pan.

In a medium bowl, whisk together flour, ¼ cup (36 grams) sesame seeds, baking soda, and 1 teaspoon (3 grams) salt.

In a large bowl, whisk together brown sugar, oil, buttermilk, 1 teaspoon (4 grams) vanilla, and eggs until smooth. Add flour mixture to sugar mixture, and whisk just until combined. Stir in banana until smooth. Spoon batter into prepared pan, smoothing top.

Bake until golden brown and a wooden pick inserted in center comes out clean, 40 to 45 minutes. Let cool completely on a wire rack.

In the bowl of a stand mixer fitted with the paddle attachment, beat cream cheese and butter at medium speed until smooth, about 1 minute. Add confectioners' sugar, remaining 1 teaspoon (4 grams) vanilla, and remaining ½ teaspoon (1.5 grams) salt, and beat until fluffy, about 1 minute. Spread three-fourths of frosting onto cake, leaving a 1-inch border on all sides. Add tahini to remaining frosting in bowl, and beat at medium speed until smooth. Dollop tahini frosting over plain frosting, and using the blade of a table knife, swirl frostings together. Cut into squares to serve.