Rub together the granulated sugar and lemon zest until combined.
Using a handheld mixer or stand mixer, beat the butter and lemon sugar until light and fluffy, about 3 minutes.
Add the egg, mix until combined. Add the milk, vanilla, and sour cream, mix until combined, about 30 seconds. Scrape down the bowl as needed.
Whisk together the flour, salt, cardamom, baking soda, and baking powder. Add it in 2 batches, mix until combined. Do not over mix. The batter will be thick.
Pour the batter into the pan, spreading it out evenly.
Top with the crumble evenly. Position it on the center rack of the oven to ensure even baking, avoiding placement too close to the top or bottom. Bake for 35-40 minutes or until a toothpick inserted comes out clean. If the cake is browning before 40 minutes, loosely cover with foil.
This cake is best enjoyed the day it’s made. If you’re enjoying leftovers the next day, simply pop each slice in the microwave for 10-15 seconds to soften the crumb, it makes a huge difference!