Preheat your oven to 350°F (175°C) and prepare an 8x8-inch baking pan with aluminum foil or parchment paper.

In a medium mixing bowl, whisk together flour, oats, salt, and baking soda until well combined.

In a large bowl, mix melted butter, brown sugar, and vanilla extract until smooth and creamy.

Gradually add the dry ingredients into the wet mixture, stirring gently to form a crumbly dough.

Reserve 3/4 cup of dough for topping and press the remaining dough evenly into the bottom of the prepared pan.

Bake for about 10 minutes or until light golden brown, then let it cool slightly.

For the filling, combine sweetened condensed milk, sour cream, egg yolk, lemon juice, lemon zest, and vanilla extract; whisk until smooth.

Pour the filling over the warm crust, spread evenly, and sprinkle the reserved crumb topping.

Bake for an additional 17 to 20 minutes, or until the top is golden brown and the filling is set.

Allow to cool in the pan on a wire rack before slicing into squares.