Preheat oven to 350℉. Lin 2 12-cup muffin tins with cupcake liners and set aside.
Streusel
Whisk together sugar, flour, and cinnamon. Add the butter and use clean hands to gently toss and then smash the butter pieces into the mixture, crumbling until the mixture looks like sand. It's okay to have some larger bits of butter as well. Set aside.
Muffins
In a large bowl, combine the cake mix, oil, cinnamon or pumpkin pie spice, eggs, sour cream, and vanilla. Using an electric mixer, beat on low for 30 seconds to incorporate the ingredients and then beat on high for 3 minutes.
In a separate bowl, toss the chopped apples in the lemon juice and then add the flour and toss to coat evenly. Fold the apples into the cake batter and then Fill muffin tins about 3/4 full. It should make 24 muffins, with possibly just a bit leftover for a solo muffin depending on the size of your tins.
Sprinkle streusel on top of muffins (about 2 teaspoons each.)
Place the pans in the preheated oven and bake for 15-20 minutes. This really depends on your oven and the moisture content of your apples so use a skewer to check for doneness. Mine take about 18-20 minutes!
Let cool slightly before enjoying, or serve at room temperature.
Notes
Store cooled muffins in an airtight container at room temperature for 1-2 days or in the refrigerator for 5-6 days for best results.
Wrap individual muffins in plastic before storing for an easy grab-and-go snack or lunch box treat.