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, beat the softened butter and granulated sugar together until light and fluffy, about 3 minutes.
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 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 melted butter to make the crumb topping.
Pour half of the batter into the prepared pan and spread evenly.
Sprinkle half of the crumb topping over the batter.
Pour the remaining batter over the crumb topping and spread evenly.
Top with the remaining crumb 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 let it cool in the pan for 15 minutes.
While the cake is cooling, prepare the lemon glaze by whisking together the powdered sugar and lemon juice until smooth.
Drizzle the lemon glaze over the warm cake.
Allow the cake to cool completely before slicing and serving.