Preheat oven to 350°F (175°C). Line an 8x8-inch pan with parchment paper.
In a bowl, whisk together melted butter and sugar until combined.
Add egg, lemon juice, vanilla extract, lemon zest, and mix well.
Stir in salt, baking powder, and flour until just combined.
Gently fold in the diced strawberries.
Spread the batter evenly into the prepared pan.
Bake for 25–30 minutes, or until the center is set and edges are lightly golden.
Let cool completely before slicing into squares.