Preheat your oven to 350°F (177°C) and line an 8-inch square baking pan with parchment paper.

In a bowl, beat together the butter and brown sugar until creamy. Add vanilla, flour, baking powder, salt, and oats; mix until crumbly.

Press more than half of the crumble mixture into the bottom of the pan to create an even layer.

Bake the crust for about 12 minutes, or until lightly golden.

Whisk the condensed milk, lemon juice, and lemon zest in a bowl until smooth.

Pour the lemon filling over the hot crust, then sprinkle the remaining crumble mixture on top.

Bake for an additional 22 to 25 minutes until edges are golden and center is set.

Let cool for at least 20 minutes before lifting out and cutting into squares.