Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round or square baking pan.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, cream the softened butter and granulated sugar together until light and fluffy using an electric mixer.
Add the eggs one at a time, beating well after each addition.
Mix in the sour cream, lemon juice, lemon zest, and vanilla extract until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
In a small bowl, combine the brown sugar, chopped walnuts, and ground cinnamon to make the streusel topping.
Pour half of the batter into the prepared pan and spread evenly.
Sprinkle half of the streusel topping over the batter.
Pour the remaining batter over the streusel layer and spread evenly.
Top with the remaining streusel topping.
Bake for 38 to 40 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool in the pan for 15 minutes.
While the cake cools, prepare the lemon glaze by whisking together the powdered sugar, lemon juice, and milk until smooth.
After 15 minutes, remove the cake from the pan and place it on a wire rack.
Drizzle the lemon glaze evenly over the warm cake.
Allow the glaze to set before slicing and serving.