Preheat oven to 350 degrees F. Line an 8x8-in baking pan with parchment paper.

Base: Melt the butter in a saucepan over medium heat. Once the butter browns slightly and becomes fragrant, remove from heat. In a large bowl, whisk together flour, rye flour, sugar, and salt. Add the melted butter and mix to combine. Transfer to the prepared baking pan, packing tightly in an even layer. Bake for 20 minutes.

Filling: While the base is in the oven, prepare the filling. Heat butter, chocolate chips, and sweetened condensed milk in a small saucepan over medium heat. Stir frequently until chocolate is melted and mixture is smooth. Remove from heat. Pour filling over baked base and use a small offset spatula to smooth into an even layer.

Topping: To make the topping, combine flour, rye flour, oats, sugar, baking powder, and salt in a mixing bowl. Use a pastry blade, fork, or your hands to cut in cold cubed butter until crumbs form. Mix in walnuts. Sprinkle topping over the chocolate layer and bake for 35-40 minutes.

Remove from oven and cool completely on a wire rack. Carefully use the parchment paper to lift from pan and cut into bars.