Preheat your oven to 350 degrees F. Line a 8×8” or 9x9" baking dish with parchment paper or spray with non-stick cooking spray.
In a small-medium bowl, combine dry ingredients (flour, baking soda, baking powder, and salt). Set aside.
In a medium bowl, add the banana and smash it.
Dice the strawberries and add to bowl with bananas along with the lemon juice.
In a large bowl, beat together butter, brown and white sugar until combined with hand mixer on medium speed.
Add in egg and vanilla and mix ingredients on high until light & fluffy.
Alternate adding the buttermilk and the dry ingredients (flour mixture) to the wet ingredients (sugar/egg batter) in mixing bowl. Mix with a whisk or spoon until combined. Tip: make sure to scrape the sides of the bowl and incorporate that into the batter.
Last, add the mashed bananas and strawberries to the batter and gently fold them in.
Pour batter (it will be thick) into greased or parchment-lined baking dish.
Bake at 350 F degrees for 35-40 minutes or until toothpick inserted in center comes out clean.
Remove from oven and let cool before frosting.