Preheat the oven to 350°F Oil an 8-by-8 inch square pan or a 9-inch round pan and dust thoroughly with flour.
Sift together the flour, baking powder, salt, and cinnamon.
Pour the tahini into a large mixing bowl and slowly drizzle in the orange juice while beating with an electric beater. Add the sugar and beat well for several minutes, until this mixture is smooth and lighter in color.
Add the dry ingredients and continue mixing with the beaters. The dough will be thick like cookie dough, so you'll have to stop and clean the beaters out when they get bogged down! Be patient, it IS worth it. :-D When it's well mixed, knead in the raisins and walnuts, if using- they ARE tasty!
Press the dough into the prepared pan, using a spatula or slightly moistened fingers to press the dough into place and smooth out the top.
Bake for 35 to 38 minutes, until light golden brown on top- be sure it's baked through, but careful not to let it dry out. Let the cake rest in the pan for several minutes, then remove from the pan and cool on a wire rack.
You have three options: eat the cake just like this (YUMMM), dust it lightly with powdered sugar, or, as Jennifer says, you can "be really Greek about it" and make a sugar syrup: Boil the sugar and water together in a small saucepan for about 10 minutes to form a slightly thick syrup. Drizzle the syrup liberally over the top of the cake and brush on the sides.
Whichever way you decide, cut this edible Greek glory into squares or wedges and enjoy!