“I thought it would taste weird because of the tahini and sesame seeds, but it was heaven and we cannot wait to make it again.” — Max, recipe tester, age 10
Adjust oven rack to middle position and heat oven to 350 degrees. Spray inside bottom and sides of 8-inch square metal baking pan with vegetable oil spray. Line bottom of baking pan with 8-inch square piece of parchment paper.
In medium bowl, whisk together flour, salt, and baking soda.
Peel bananas and place in large bowl. Use large fork or potato masher to mash bananas until broken down but still chunky. Measure out 1 cup mashed bananas (discard any extra).
In bowl of stand mixer (or large bowl if using handheld mixer), combine softened butter, sugar, and tahini. Lock bowl into place and attach paddle to stand mixer, if using. Start mixer on medium-high speed. Beat until mixture is light and fluffy, 3 to 4 minutes.
Add eggs and beat until combined, about 30 seconds. Add mashed bananas and vanilla and beat until incorporated, about 30 seconds. Stop mixer.
Carefully add half of flour mixture. Start mixer on low speed and mix until combined, about 1 minute. With mixer running, slowly pour in milk and mix until combined, about 30 seconds. Stop mixer.
Add remaining flour mixture. Start mixer on low speed and mix until well combined, about 1 minute. Stop mixer. Remove bowl from stand mixer, if using.
Use rubber spatula to scrape down sides of bowl and stir in any remaining dry flour. Use rubber spatula to scrape batter into parchment-lined baking pan and smooth top (make sure to spread batter into corners to create even layer). Sprinkle sesame seeds over top.
Place baking pan in oven. Bake until cake is deep golden brown and toothpick inserted in center comes out clean, 40 to 50 minutes.
Use oven mitts to remove baking pan from oven (ask an adult for help). Place baking pan on cooling rack and let cake cool completely in pan, about 2 hours.