In a bowl, combine the flour, cornmeal, baking powder, baking soda, and salt.
In the bowl of an electric mixer, beat the butter and 1 1/4 cups of the granulated sugar on medium-high until smooth. Add the eggs and beat until fluffy, 2 minutes. Add the sour cream, vanilla, lemon zest, and poppy seeds and beat to combine. Reduce speed to low and slowly incorporate the flour mixture.
Place a 15-inch piece of parchment paper in the bowl of a slow cooker, letting the excess come up the sides. Transfer the batter into the bowl of the slow cooker. Set the cooker to high and cook, covered, until set and a skewer inserted in the center comes out clean, about 2 1/2 hours.
Combine the lemon juice and the remaining granulated sugar in a bowl and drizzle evenly over the top of the cake. Holding the parchment paper, transfer the cake to a rack. Let cool for at least 15 minutes. Dust with the confectioners' sugar and serve with the whipped cream, if desired.
If You Don't Have a Slow Cooker: Heat oven to 350° F. Follow the recipe above using a buttered 9-by-13-inch pan. Bake until set, about 45 minutes.