In a large bowl, beat the softened butter and sugar for 2 minutes, until creamy. Beat in the egg yolk and vanilla, then add the flour and salt and mix on low speed until a soft dough forms.
Preheat the oven to 350°F and line a baking sheet with parchment paper. Roll the dough into 18 balls, about 1 tablespoon each, and place them 2 inches apart.
Press a deep, wide well into the center of each ball with your thumb or a rounded 1/2 teaspoon measure, then chill for 15 minutes. While the dough chills, blend the cottage cheese, sugar, egg white, cornstarch, vanilla and lemon zest until completely smooth and silky.
Bake the cookie shells for 8 minutes. Remove from the oven and gently press the wells again with the back of a teaspoon while the cookies are still warm.
Fill each well with the cheesecake filling almost to the rim. Add a small dot of strawberry jam on top and swirl it lightly with a toothpick.
Return to the oven and bake for 10 to 12 minutes, until the filling is set and the edges are lightly golden.
Cool on the baking sheet for 10 minutes, then move to a wire rack. Chill for 1 hour for the creamiest cheesecake centers, and store in the fridge.