In a large bowl, mix the yogurt and sugar. Dissolve the baking soda in the water and add to the mixture. Then add the mastic and semolina, little by little, mixing until smooth.
Thoroughly grease a baking tin, pour the mixture into it and bake the samali at high heat until golden brown (30-40 minutes). Meanwhile, prepare the syrup by boiling the sugar, water and lemon peel for 10-15 minutes.
Take the cake out of the oven, and cut into slices while it's still hot. Melt the leftover butter and pour over the cake. Then add the warm syrup. Leave to stand for several hours, so the syrup is absorbed into the cake.