Using a handheld mixer or stand mixer, beat the butter and 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. The mixture might look lumpy, that's okay.
Whisk together the flour, salt, baking soda, and baking powder. Add it in 2 batches, mix until combined. Do not over mix.
Pour the batter into the pan, spreading it out evenly.
Top with the crumble evenly (break it up if needed) 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 done, loosely cover with foil.
Set aside to cool before topping with caramel.
Drizzle with salted caramel when cooled down. I used about ½ a cup, you can drizzle with as much as your heart desires! Recipe linked in notes
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!