Preheat the oven to 180 degrees C. Grease a 22x33cm baking dish.
For the tahini swirl, combine the yoghurt, tahini, sugar, egg and milk in a medium bowl and mix until smooth. Set aside.
For the brownie mixture, mix together melted butter and sugar, then whisk in the eggs one at a time, beating well after each addition. Combine flour, cocoa, cinnamon, baking powder and salt; gently mix into the butter mixture.
Remove and reserve 1 cupful of the chocolate mixture. Spread the remaining chocolate mixture into the prepared tin (it will be very thick). Spread the tahini swirl mixture over the top, all the way to the edges. Drop the reserved chocolate mixture by small teaspoonfuls evenly over the tahini mixture. Using a knife, gently swirl through the top layers for a marbled effect.
Bake in preheated oven for 35 to 40 minutes, or until a skewer inserted near the centre comes out almost clean. Cool completely, then cut into squares. (For best results, chill in the fridge before slicing.)
Brownies have the best texture when ever so slightly under-baked. Remove from oven when your tester is almost clean; some crumbs left on the tester is ideal.