Mix the streusel ingredients in a small bowl until well combined. ***
Preheat the oven to 350°F / 175°C. Line a muffin tin with 9 muffin liners.
In a medium bowl, whisk together the dry ingredients (flour through salt).
In another bowl, mix the wet ingredients (egg through yogurt).
Add the flour mix to the wet mix. It just needs to be combined, so be careful not to overmix.
Then fold in the apples, being careful not to overmix.
Fill the liners with the batter, filling them about 2/3 full.
Divide the streusel between the muffins. If you like being exact, that's 20 grams per muffin for the wheat version and 23 grams for the GF version. It's a little more than a tablespoon.
Bake for about 15 – 17 minutes or until a toothpick comes out clean. I needed 15 for the wheat version and 17 minutes for the GF version. But all ovens vary so have an eye on them.
Let the muffins cool for 5 minutes and then turn out onto a wire rack to cool.
When the muffins are completely cooled, place them in an airtight container and store at room temperature for up to 4 days. Anything past that, store in the refrigerator.