Preheat your oven to 325°F. Grease a 9x5 inch loaf pan and line it with parchment paper, leaving an overhang on the long sides.
In a bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt.
In a large bowl, beat the butter and sugar for 4-5 minutes until very pale and fluffy. This creaming step creates the lift, so do not rush it.
Beat in the yogurt and vanilla until combined. The mixture may look slightly curdled, and that is completely normal.
Add the dry ingredients in two additions, alternating with the milk, mixing on low just until no flour streaks remain.
Spread the batter into the pan and bake for 55-65 minutes, until a toothpick comes out clean. Tent loosely with foil after 40 minutes if the top is browning too fast.
Cool in the pan for 15 minutes, then lift out and cool completely on a rack before slicing.
