Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper.

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

In another bowl, melt butter and let it cool slightly. Stir in brown sugar and vanilla until smooth.

Combine wet and dry mixtures to form a thick, crumbly dough. Reserve ¾ cup for topping.

Press the remaining mixture evenly into the prepared pan to form the crust.

Bake the crust for about 10 minutes until set but not browned.

In a clean bowl, whisk together sweetened condensed milk, sour cream, egg yolk, lemon juice, zest, and vanilla until smooth.

Spread the creamy filling over the warm crust and sprinkle the reserved crumb mixture on top.

Bake for an additional 17 to 20 minutes until lightly golden.

Allow the bars to cool completely on a wire rack before slicing.

Once cooled, lift the bars out and slice into squares to serve.