Preheat the oven to 350°F (175°C). Line a square baking pan with parchment paper, leaving extra paper on the sides for easy lifting later.

In a large bowl, combine flour, oats, brown sugar, salt, and baking powder. Stir well until the mixture is evenly blended and no lumps remain.

Pour melted butter into the dry ingredients. Mix gently until a crumbly texture forms with small clumps and loose crumbs.

Take half of the crumble mixture and press it firmly into the bottom of the prepared pan. Spread it evenly to create a solid base layer.

In a separate bowl, mix crushed pineapple, sugar, cornstarch, and vanilla extract. Stir until the mixture becomes slightly thick and well combined.

Spread the pineapple filling evenly over the base layer, making sure it covers the surface completely.

Sprinkle the remaining crumble mixture over the pineapple layer in small clumps. Do not press it down.

Bake for 30 to 35 minutes, or until the top is golden brown and the filling looks set.

Remove from the oven and allow the bars to cool completely in the pan. This helps them firm up for clean slicing.

Lift the bars out using the parchment paper and cut into squares before serving.