Grease the inside of your slow cooker generously with butter or oil. Line with parchment paper if desired for easy removal, leaving an overhang.

In a medium bowl, whisk together the flour, salt, baking powder, and baking soda. Set aside.

In a large mixing bowl, beat the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes. This step incorporates air for a tender crumb.

Beat in the eggs one at a time, ensuring each is fully incorporated. Stir in the vanilla extract, lemon extract, fresh lemon juice, and rub in the lemon zest.

Gradually add the dry mixture to the wet, alternating with the sour cream and milk. Start and end with dry ingredients. Mix until just combined to avoid toughness.

Pour the batter into the prepared slow cooker. Cover and cook on LOW for 4 hours or until a toothpick inserted in the center comes out clean (it may take up to 5 hours depending on your slow cooker). Do not open the lid frequently.

Turn off slow cooker and let the loaf cool in the cooker for 15-20 minutes with lid off. Carefully remove and place on a wire rack. Poke small holes over the surface with a toothpick while warm.

Whisk together powdered sugar, lemon juice, and milk until smooth and thick enough to drizzle.

Pour glaze over the warm loaf. Let set completely before slicing.