Preheat oven to 325°F.
In a bowl, mix together flour, salt, and baking soda.
Using a hand mixer (or stand mixer), cream butter with sugar and lemon zest, until light and fluffy (about 8 mins). Add the sour cream, mix until combined. Then beat in the eggs and egg yolk 1 at a time. Add the lemon juice, mix until well combined.
*don't forget to scrape down the sides of the bowl between additions.
Lower the speed and add dry ingredients, mix until just incorporated. Then finish mixing with a spatula. The dough will be thick.
Grease 9x5 inch loaf pan with butter and cover the bottom of the pan with parchment paper (сover only the long sides and leave some hanging paper on each side, it will help to unmold the cake).
Transfer the cake dough to the baking pan. Bake in the preheated oven for 95 mins, or until a toothpick inserted into the cake comes out clean. While the cake is baking, you can prepare a syrup.
Combine sugar, lemon juice, and water in a saucepan. Bring to a boil over medium heat and continue to a low boil for 5 minutes, stirring occasionally. Remove from heat and set aside.
Allow the cake to cool in the pan for 10 mins. Poke holes all over the cake with a skewer. Evenly pour half of the syrup over the cake. When the syrup is soaked up, repeat with the remaining syrup.
Remove cake from the pan using hanging parchment paper. Let it cool completely in a wire rack.
To make an icing: in a bowl, combine melted butter, powdered sugar, and lemon juice. Pour the lemon icing over the cake. Slice and enjoy it!