Use a microplane or box grater to zest 2 large lemons. Juice and strain one lemon until you have juice.
Whisk the flour, baking powder and salt together in a medium bowl.
In a large bowl, cream the butter, Crisco and sugar until it's light and fluffy. Beat in the eggs one at a time, making sure each is well-incorporated. On low speed, mix in the lemon juice and lemon zest.
On low speed, gradually blend in the dry ingredients until just combined. Cover and chill in the refrigerator for at least two hours. (You can also chill the dough overnight.)
Roll out half of the chilled dough onto a floured surface until an 1/8 to 1/4 inch thick. Keep the rest of the dough in the refrigerator until you're ready for it. Use cookie cutters to cut out shapes and transfer them onto a baking sheet lined with parchment paper or a silicone baking mat. Don't put more than 12 cookies on a pan.
Re-roll the scraps until you've used up the dough and then roll out the remaining chilled dough.
Chill each sheet of cookies for at least 30 minutes before baking. Preheat the oven to 375°F (190°C), and then bake the cookies (one pan at a time) for no more than 6-7 minutes. Don't worry if they still look a little soft when you take them out of the oven. (But you don't want them to look wet and doughy.) Thicker, larger cookies will need 7 minutes, and smaller cookies will only need 6.
Leave the cookies on the baking sheet for 5 minutes before transferring them to a wire cooling rack.
Make the frosting while the cookies are cooling. (I actually recommend freezing the cookies before decorating them.) Cream the butter and Crisco until smooth. Mix in half of the powdered sugar on low speed. Then add the milk, heavy cream, lemon juice and the remaining powdered sugar. Mix on low speed until just combined, and then beat on high speed until smooth and fluffy. Double the frosting recipe if you plan on using piping tips to decorate. If you're just spreading the frosting on with a knife then this is enough.
Decorate as desired and store in an airtight container in the freezer. Enjoy!