Preheat oven to 350 degrees. Grease a standard 8 inch loaf pan and line it with parchment paper.
In a mixing bowl, stir together the flour, baking powder, and salt until combined. In another bowl, whisk together the sour cream, 1 cup of the sugar, the eggs, lemon zest, and St. Germain until smooth. Add the flour mixture to the wet ingredients and mix until incorporated. Add the oil and whisk until a smooth batter forms. Pour the batter evenly into the prepared loaf pan. Bake 60 minutes, or until golden and cracked on top and set in the center. Allow the cake to cool 10 minutes before using the parchment paper to lift the cake out of the pan.
Meanwhile, in a small saucepan over medium heat, whisk together the remaining 1/3 cup of sugar and the lemon juice. Cook until the sugar has completely dissolved. When the cake has cooled 10 minutes and has been removed from the pan, place it onto a cooling rack. Using a standard toothpick, prick small holes all over the top and sides of the cake. Pour the lemon sugar syrup evenly over the cake. Allow the syrup to soak in as the cake cools.
When the cake has cooled, whisk together the powdered sugar and St. Germain in a mixing bowl until a thick glaze forms. Drizzle the glaze over the cake. Slice and serve.