Melt 3 tablespoons of butter in a skillet over medium heat. Saute the leeks, stirring, until wilted. Add the kale and chicken stock. Cover the pan and reduce the heat to low; simmer for about 30 minutes, or until the vegetables are very tender. Add the salt, to taste, and the black pepper. Add the half-and-half or cream; remove from heat and set aside.

Meanwhile, peel the potatoes and cut them into chunks. Rinse the potatoes and put them in a medium saucepan; cover with water and add the remaining 1 teaspoon of salt.

Bring the potatoes to a boil over high heat. Reduce heat to medium-low, cover the pan, and continue cooking until tender, about 20 minutes. Drain the potatoes thoroughly and transfer to a large bowl.

Heat the kale and cream mixture until hot. Add to the drained potatoes along with the remaining 2 tablespoons of butter and mash by hand or with a mixer.

Taste and adjust seasonings.