Combine flour, cornmeal, baking powder, baking soda, and salt in a medium bowl. Set aside.
Using an electric mixer, beat butter and sugar until smooth, about 2 minutes.
Add eggs and beat another 2 minutes.
With mixer on low, mix in sour cream, lemon zest, vanilla extract, and poppy seeds.
Slowly add flour mixture and mix well.
Pour batter into parchment lined slow cooker. Cover and cook on HIGH for 2 hours 15 minutes to 2 hours 30 minutes. Cake should be set in center.
Remove cover and turn crock pot off.
In a small bowl, whisk together lemon juice and confectioners' sugar.
Remove crock pot insert from crock pot and place on a wire rack to cool. Drizzle lemon/sugar mixture on top.
Notes
When you zest the lemon, make sure you only remove the yellow part and not the white part. The white part is bitter.If you have too much condensation building up and falling back down on the middle of the cake, you can place a tea towel or a few paper towels under the lid to catch it.