Adjust oven rack to middle position and heat oven to 350 degrees. Grease 8-inch square baking pan. Whisk flour, salt, and baking soda together in bowl.

Using stand mixer fitted with paddle, beat butter, tahini, and sugar on medium-high speed until light and fluffy, about 3 minutes, scraping down bowl as needed. Add eggs, one at a time, and beat until combined. Add bananas and vanilla and beat until incorporated. Reduce speed to low and add flour mixture in 3 additions, alternating with milk in 2 additions, scraping down bowl as needed. Give batter final stir by hand.

Transfer batter to prepared pan and smooth top with rubber spatula. Sprinkle top with sesame seeds. Bake until deep golden brown and toothpick inserted in center comes out clean, 40 to 50 minutes, rotating pan halfway through baking.

Let cake cool in pan on wire rack for 10 minutes. Remove cake from pan, discarding parchment, and let cool completely on rack, about 2 hours. Serve. (Cake can be stored at room temperature for up to 2 days.)