Preheat oven to 375 degrees Fahrenheit.

To make the streusel, add flour, light brown sugar, cinnamon, salt, and cubed cold butter to a bowl. Crumble the mixture together with your fingers until a “sandy” texture is formed. There should be some pea sized crumbs of streusel.

To make the buttermilk, add one tablespoon of white vinegar to a one cup measuring cup. Fill the cup the rest of the way with milk. Let it sit for five minutes- it will look curdled and add a nice tang and flavour to the muffins!

In a large mixing bowl, sift together the cornstarch and flour. Add the sugar, baking powder, and salt, and whisk to combine.

Make a well in the centre of the dry ingredients, and to the well add the buttermilk, oil, egg, and vanilla extract. Whisk lightly with a whisk to break up the egg, switch to a spatula, and gently fold the dry into the wet ingredients in the centre. Mix the batter just until combined.

Lightly spray muffin liners with nonstick spray. Fill each about a quarter full with batter, add some blackberries, and more batter on top. Top with another blackberry and a generous heaping of streusel.

Bake for 25–30 minutes (depending on your oven), until muffin tops are set.