Preheat oven to 350°F (180°C) and grease a 9×5-inch loaf pan.
In a large bowl, whisk together yogurt, sugar, eggs, and oil until smooth.
In a separate bowl, stir flour, baking powder, and salt.
Scoop ⅓ of the batter into another bowl and mix in cinnamon.
Alternate spoonfuls of plain and cinnamon batter into the pan, then swirl with a knife.
Mix sugar, flour, and cinnamon, then cut in butter until crumbly.
Sprinkle topping evenly and bake 45 minutes, or until a toothpick comes out clean.
Let cool in pan 10 minutes, then transfer to a rack.
