Preheat the oven to 425°F. Line a baking tray with a silpat liner or parchment paper.
Add the milk to a small bowl and drizzle the vinegar on top. Let it sit without stirring for 10 minutes to curdle*.
Whisk together the flour, cornstarch, sugar, baking powder, salt, and baking soda in a large bowl.
Add the butter to the dry ingredients and use your hands or a pastry cutter to cut in the butter until it forms a coarse meal (coarse crumbs). If you’re using your hands, rub the butter into the flour with your fingertips, leaving some pieces of butter the size of peas.
Add the curdled milk to the dry ingredient/butter mixture, and use a rubber spatula to stir it in. You will end up with a shaggy dough, and some streaks of flour are fine.
Divide the dough into 4 equal portions, roll each into a ball, and arrange them on the prepared baking tray. Sprinkle the 1 teaspoon of sugar on the tops.
Bake until the shortcake is puffed and light golden, about 12 to 15 minutes. Cool.