Preheat the oven to 350°F (175°C) and prepare a 9x9 inch baking pan with foil or parchment paper.

In a medium bowl, whisk together the all-purpose flour, oats, salt, and baking soda until fully combined.

In a separate bowl, mix melted butter, brown sugar, and vanilla extract until smooth. Gradually incorporate the dry ingredients until a crumbly dough forms.

Set aside 3/4 cup of the dough for the topping. Press the remaining dough evenly into the base of the prepared pan.

Bake the crust for about 10 minutes or until lightly golden.

Whisk together sweetened condensed milk, sour cream, egg yolk, lemon juice, lemon zest, and vanilla extract in a large bowl until smooth.

Pour the creamy lemon filling over the warm crust and spread evenly.

Crumble the reserved dough evenly over the lemon filling.

Bake for an additional 17-20 minutes until the topping is golden brown and the filling is set, but slightly jiggly in the center.

Let the pan cool completely on a wire rack, then slice into squares.