In another large bowl, add the applesauce (or vegetable oil), eggs, sugars, and vanilla and mix with a whisk until fully combined.
1 cup unsweetened applesauce, 3 eggs, 1 1/2 cups sugar, 1/2 cup brown sugar, 1 teaspoon vanilla extract
Pour the dry ingredients into the wet ingredients in batches (I did three parts) and stir with a spatula until fully combined. The batter will be very thick.
Stir in the diced apples.
Pour half of the batter into each prepared loaf pan – try to keep the amounts even.
To make the streusel
In a small bowl, add the softened butter, cinnamon, and sugar and mix together with a fork. Drop the streusel mixture onto the tops of the batter and lightly swirl.