1. Prepare the Pineapple Filling:

1. In a small saucepan, combine the crushed pineapple, brown sugar, and cornstarch.
2. Cook over medium heat, stirring constantly, until the mixture thickens (about 3-5 minutes).
3. Remove from heat and let it cool completely.

2. Make the Batter:

1. Preheat your oven to 350°F (175°C) and grease a 9×9-inch baking dish or line it with parchment paper.
2. In a large mixing bowl, combine the melted butter and granulated sugar. Mix until smooth.
3. Beat in the eggs, one at a time, then add the vanilla extract.
4. In a separate bowl, whisk together the flour, baking powder, and salt.
5. Gradually fold the dry ingredients into the wet mixture until just combined.

3. Assemble the Cake Bars:

1. Spread half of the batter evenly into the prepared baking dish to form the bottom layer.
2. Spoon the cooled pineapple filling evenly over the batter.
3. Top with the remaining batter, spreading it gently to cover the filling.

4. Bake and Cool:

1. Bake in the preheated oven for 25-30 minutes, or until the top is golden and a toothpick inserted into the center comes out clean.
2. Let the bars cool completely in the pan before slicing into squares.