In a medium bowl, stir together ⅔ cup all-purpose flour, ¼ teaspoon ground cinnamon and ⅓ cup white sugar.
Drizzle ¼ cup unsalted butter, melted and lukewarm, and stir with a fork, spatula or spoon to integrate. The mixture will be crumbly and irregular, with flour patches.
Refrigerate while making the muffin batter.
For the apple muffins:
Preheat the oven to 350ºF (180°C).
Line 6 jumbo muffin pans or 12 regular muffin tins with paper liners, or butter/spray the pans.
In a large bowl, sift together the dry ingredients: 2 cups all-purpose flour, ⅓ cup white granulated sugar, ⅔ cup light brown sugar, 1 tablespoon baking powder, ½ teaspoon salt and 1 teaspoon apple pie spice. Stir to combine.
Whisk 2 eggs, ¾ cup buttermilk, ½ cup vegetable oil, and ½ teaspoon vanilla extract in a medium bowl. Do not beat, but mix well until no streaks of egg whites remain.
Add the wet ingredients at once to the bowl with the dry ingredients.
Mix until you barely have streaks of flour, but do NOT overmix. It will be lumpy and not completely mixed and that is fine.
Add 2 cups chopped Granny Smith apples and stir lightly to combine with a rubber spatula.
Divide the apple muffin batter evenly among the prepared muffin cups or liners. Don't go over ¾ of their capacity. Sprinkle the cold streusel topping over each muffin. If necessary, break up large clumps. You might not use it all.
Bake for 25-30 minutes, until the top is dry and golden brown, and a cake tester or toothpick inserted in the center comes out clean. Don't overbake.
Let cool down completely before glazing. Or eat warm without glaze.
They are best eaten the day they are made, but you can keep them for a day at room temperature, covered or wrapped. Store leftovers, covered in plastic wrap or in an airtight container, in the refrigerator for a few days or frozen for a month.
For the lemon glaze:
Mix ½ cup powdered sugar with 1 tablespoon lemon juice in a small bowl until smooth and creamy. It should be like thick honey. If needed, add more lemon juice, ½ teaspoon at a time, until you reach the desired consistency.
Drizzle over the cooled muffins and let them set before eating.
Video
Notes
Homemade buttermilk: mix ¾ cup of milk (preferably whole milk) with 1 tablespoon of lemon juice. Stir and let the mixture sit for 5 minutes at room temperature until it thickens and slightly curdles. Stir it once more, and measure it again before using.Extra citrus flavor: use orange or lemon. Add a teaspoon of orange or lemon juice and ½ teaspoon of zest to the batter. Orange is mellower and sweeter, but lemon is sharp, so go easy with the zest.