Pour the buttermilk into a 2-cup measuring cup, and stir in the poppy seeds. Leave set at room temperature for at least 2 hours.
Preheat, the oven to 350 degrees F. Spray the inside of a Bundt pan with the baking spray making sure to coat thoroughly then set aside.
In the bowl of an electric mixer with a paddle attachment, cream the butter and 2 cups of sugar on medium speed for about 5 minutes, until light yellow and fluffy.
Reduce mixer speed to low, add in the eggs, one at a time, mixing until incorporated after each egg. Add the vanilla, and lemon zest, scraping down the bowl with a spatula.
Sift the flour, cornstarch, salt, baking powder, and baking soda into a medium bowl.
Add 1/4 cup of the lemon juice to the buttermilk/poppyseed mixture.
With the mixer on low speed, alternately add the flour mixture and buttermilk mixture in thirds, beginning and ending with the flour.
Scrape the bowl with a spatula to be sure the batter is all well mixed.
Place the batter into the prepared pan, smoothing the top, and bake for 40 to 50 minutes, until a cake tester comes out clean.
While the cake is baking place the remaining 1/2 cup of granulated sugar and the remaining 1/2 cup of lemon juice in a small saucepan and cook over high heat until the sugar dissolves. Set aside.
When the cake is done, allow to cool in the pan for 10 minutes. Then turn the cake out onto a baking rack set over a sheet of parchment paper.
Brush the warm lemon syrup slowly over the cake, letting it be absorbed into the cake. Set aside for at least 30 minutes to cool.
To make the glaze, whisk the confectioners’ sugar and lemon juice together in a small bowl, adding a little more sugar or lemon juice to make a smooth, thick glaze.
Drizzle over the cake, allowing it to drip down the sides. Transfer to a flat cake plate and garnish with lemon zest and candied lemons if desired.
Serve cake at room temperature.