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

In a medium bowl, whisk together 1 cup of flour, 1 cup of oats, 1/2 teaspoon of salt, and 1/2 teaspoon of baking soda.

In a large bowl, whisk together 1/2 cup melted unsalted butter, 1 cup brown sugar, and 1 teaspoon vanilla extract until smooth.

Gradually add the dry mixture into the wet mixture and stir until just combined. Reserve about 3/4 cup of this mixture for topping.

Press the remaining mixture into the bottom of the lined baking pan and bake for about 10 minutes until firm and lightly golden.

In a medium bowl, whisk together 1 can of sweetened condensed milk, 1/2 cup sour cream, 1 egg yolk, 1/3 cup lemon juice, zest of 1 lemon, and 1 teaspoon vanilla extract until creamy.

Pour the lemon filling over the warm crust, spread evenly, and sprinkle the reserved crumb mixture on top.

Bake again for 17-20 minutes until lightly golden and the filling is set but slightly jiggly in the center.

Cool completely on a wire rack, then lift the bars out and cut into squares.