Preheat the oven to 350 F, grease 9 x 5-inches loaf pan with baking spray, line the bottom with parchment paper leaving large overhang the sides, and set aside.
To make the crumb topping, stir together flour, sugar, brown sugar, and salt. Add melted butter and whisk with a fork until coarse crumbs form. Set aside.
To make the cream cheese filling, mix softened cream cheese, sugar, vanilla (or lemon) extract, egg white and flour just to combine.
To make the lemon cake mixture whisk dry ingredients (flour, baking powder and salt) and set aside.
In a large bowl mix softened butter and sugar for about 2 minutes at high speed. Add eggs, lemon zest, vanilla, and lemon extract and mix for 2 more minutes. Add sour cream and lemon juice and mix just to combine. Running your mixer on low, mix in dry ingredients.
To assemble the loaf, spread about ½ half of lemon cake mixture into the bottom of prepared pan. Using a spoon dollop the cream cheese mixture over the cake layer, but do not go all the way to the edges. Using the back of the spoon, gently smooth the top. Cover cream cheese filling with cake mixture. The cake batter is thick so it is the best to dollop over the top, then gently spread, again using the back of the spoon.
Sprinkle streusel crumbs on top.
Place in preheated oven and bake about 56-70 minutes or until the center has set. Tent the top loosely with aluminium foil if it starts browning too much.
Cool 5-10 minutes in the pan, then transfer on a rack to cool completely. Use the overhangs of the parchment paper to lift the loaf from the pan.
Meanwhile, prepare the glaze. Stir powdered sugar with lemon juice. Start with 1 teaspoon of the juice and gradually add more until desired consistency has reached. Drizzle over the cake and serve.
