In a mixer fitted with a paddle, mix the flour and salt in a large bowl. Add the butter and mix the ingredients together into a sandy, crumbly mixture. In a bowl, combine the sour cream and egg yolk until very smooth. Add to the flour mixture and mix until the dough comes together; the dough may feel a little dry. Form into a disk, wrap it in plastic wrap and chill at least 4 hours or overnight.
In a mixing bowl with a whisk attachment, cream the cream cheese. Add the egg, powdered sugar, vanilla and lemon zest.
On a work surface thickly dusted with powdered sugar, roll out the pastry to a squarish or rectangular shape, 1/8-inch thick. Using the tip of a sharp knife or a pizza cutter, cut into 3-inch squares. Place a teaspoon of cream cheese filling in the center of each square. Lift up 1 square, gathering the center of the square around the filling into a cup. Hold it in the fingers of one hand. Use the fingers of your other hand to gently push the midpoints of the sides inward, so that the four corners become the points of the petals. The cookie will look like an opening flower. Place the cookie in a cup of a mini muffin pan. Repeat with the remaining squares and refrigerate for 30 minutes.
Preheat the oven to 375°. Bake the lemon buttercups until light golden brown, 25 to 30 minutes. Let cool in the pan on a wire rack.