Heat oven to 350º F (180º C) and line a 13x9" (33 cm x 23 cm) baking pan with parchment paper.
Combine brown sugar and applesauce together by hand until in a medium bowl.
Add banana, yogurt, eggs and vanilla. Whisk by hand to combine.
In a separate large bowl, combine flour, ground flax seed, baking soda, salt, and cinnamon.
Pour the wet ingredients into the dry ingredients. Mix until just combined.
Stir in oats and chocolate chips.
Spread dough onto the prepared 13 x 19 inch (33 cm x 48 cm) baking pan.
Bake in the pre-heated oven for 22-25 minutes or until an inserted toothpick comes out clean.
Cool on a rack in the pan for about 15 minutes. Once cooled, cut into 24 bars.