In a large bowl, whisk together the flour, baking powder, kosher salt, and granulated sugar.
In a medium bowl, mash the banana until smooth. Add the buttermilk, egg, and cooled melted butter. Whisk until well combined.
Pour the wet ingredients into the flour mixture. Stir gently until just incorporated. (A few lumps are fine; do not overmix.)
Gently fold in the diced strawberries.
Spray a nonstick skillet or griddle with cooking spray and heat over medium heat (about 300°F).
When hot, pour about ⅓ cup of batter onto the skillet. Spread the batter slightly to about a 4-inch diameter, if needed.
Cook for 3-4 minutes, until bubbles begin to pop on the surface and the bottoms are golden brown.
Flip gently and cook an additional 3-4 minutes, until golden brown on the second side.
Repeat with remaining batter, wiping the skillet with a paper towel between batches if needed.
Serve warm with maple syrup and whipped cream, or your favorite toppings.