Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Combine tahini, yogurt, sugar, egg, and milk in a medium bowl; mix until smooth.
Combine melted butter and sugar in a bowl and mix until well combined. Whisk in eggs one at a time, beating well after each addition.
Combine flour, cocoa powder, cinnamon, baking powder, and salt in a bowl. Gently mix into the butter mixture until batter is thick and smooth.
Remove 1 cup of the batter and set aside. Spread remaining batter into the prepared baking pan. Pour tahini mixture on top and spread all the way to the edges of the pan. Drop small teaspoonfuls of the reserved batter evenly over the tahini mixture. Gently swirl a knife through the tahini layer to create a marbled effect.
Bake in the preheated oven until a skewer inserted in the center comes out almost clean, 35 to 40 minutes. Remove from oven and cool completely before slicing into squares.
