Prepare the crumb topping by blending cold butter cubes with flour, sugar, and lemon zest until the mixture forms coarse crumbs. Set aside in the fridge to keep chilled.
Cream softened butter and sugar in a large bowl until light and fluffy, then beat in the eggs one at a time, ensuring each is fully incorporated.
Combine dry ingredients flour, salt, baking powder, and baking soda—in a separate bowl, mixing well to evenly distribute the leavening agents.
Alternate adding dry ingredients and milk to the butter mixture, beginning and ending with the dry ingredients. Mix gently to keep the batter tender.
Fold in sour cream, lemon juice, lemon zest, and vanilla extract until the batter is smooth and fragrant, careful not to over mix.
Transfer the batter to a greased baking pan, spreading it evenly. Sprinkle the chilled crumb topping generously over the surface.
Bake at 350°F (175°C) for about 40 minutes, or until a toothpick inserted in the center comes out clean and the crumb topping is golden.
Mix the icing ingredients until smooth, then drizzle over the cooled cake for a tangy, sweet finish that complements the lemon flavor perfectly.