In a medium bowl, stir together ⅔ cup flour and ⅓ cup sugar.
Add ¼ cup unsalted butteritter and work with your hands or a pastry cutter to integrate. The mixture will be crumbly and irregular, with butter pieces the size of peas and beans.
Refrigerate while making the muffin batter.
For the apple muffins:
Preheat the oven to 350ºF (180°C).
Line 6 jumbo muffin pan 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 , ⅓ cup white granulated sugar, ⅔ cup light brown sugar, 1 tablespoon baking powder, ½ teaspoon salt and 1 teaspoons 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 apple 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 with the streusel topping. 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 dry before serving.
Notes
Baking time: keep in mind that all ovens and pans are different, even if they look the same or very similar. The baking time in my recipes is as accurate as it can be, but it might take you more or less time. You can use a thermometer to check that your oven is at the right temperature. I recommend you keep track of how your oven works and what tiny details you might need to adjust. Homemade buttermilk: mix 1 cup of milk (preferably whole milk) with 1 tablespoon of lemon juice or white vinegar. 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.