Equipment & prep
Preheat the oven to 350°F (177°C). Line an 8-inch square pan with parchment paper so you can lift the finished slab out easily.

Make the oat mixture
In a mixer (or with a sturdy spoon), cream the softened butter with the brown sugar until smooth. Mix in the vanilla. Add the flour, baking powder, salt, and oats and stir until the mixture looks like coarse crumbs. You should end up with roughly 3 to 3½ cups of crumbly mixture. Reserve about one cup for the topping; press the rest firmly into the bottom of the prepared pan to form the crust.

Blind-bake the crust
Bake the pressed crust 12 minutes, until it’s just beginning to color. Remove from the oven and leave the oven on.

Prepare the lemon filling
Whisk the sweetened condensed milk together with the lemon juice and lemon zest until glossy and uniform. The acid will slightly thicken the mixture; that’s normal.

Assemble and top
While the crust is still warm, pour the lemon mixture over it and spread evenly. Sprinkle the reserved oat crumble evenly across the lemon layer.

Final bake
Return the pan to the oven and bake for 22–25 minutes, or until the edges turn a light golden brown and the center is set (it shouldn’t jiggle). Take the pan out and cool it on a rack until it comes to room temperature.

Finish and slice
Lift the parchment out of the pan and place the slab on a cutting board. Use a sharp knife (wipe between cuts for clean edges) and slice into squares.

Storage & freezing
Store the squares at room temperature for up to 2 days, or refrigerate in an airtight container for up to 1 week. To freeze, arrange cut pieces in a single layer separated by parchment in a freezer-safe container; they’ll keep up to 3 months. Thaw overnight in the fridge, then bring to room temp before serving.