In a medium bowl, mix together the all-purpose flour, rye flour and salt. Set aside.
In a large bowl (or the bowl from your stand-up mixer), add the room temperature butter, créme fraiche (or sour cream), milk and light brown sugar. Beat until the light and fluffy, about 5 minutes. Pour in the vanilla extract and beat until combined. Turn the mixer on low, and in two batches, add the flour mixture; mix until combined. If the dough looks a bit crumbly, not to worry---it'll come together when we start kneading. Keeping the dough in the bowl, knead the dough with your hands until it forms a big uniform ball. Divide the dough into two mounds; wrap each dough ball in plastic wrap and transfer to the refrigerator for 2 hours or overnight.
Preheat the oven to 350 degrees F. Remove the first ball of dough from the refrigerator and place it in the center of a sheet of parchment. Top the dough with a second piece of parchment and using your rolling pin, roll the cookie dough to a 1/4-inch thickness.
Flour your cookie cutter so it doesn't stick and cut the shapes as close to one another as possible (this is so we can avoid re-rolling the scrapsthey never taste as great). Using a flat spatula, carefully transfer the cookie dough to a parchment-lined baking sheet. Bake for 6 to 7 minutes. Allow the cookies to cool for a few minutes on the baking sheet and then carefully transfer the cookies to a cooling rack until room temperature. When the cookies are warm they're a bit fragile so be gentle!
You can dust the cookies with powdered sugar, if you like and serve immediately. If you're going the icing route, let's get to it!
In a large bowl, add the egg whites. Using the beaters from an hand-mixer or whisk attachment from a stand-up mixer, beat the egg whites until frothy, about 2 minutes. In two batches, add the sifted powdered sugar, until the sugar is incorporated. Turn the mixer to high and beat the mixture until it doubles in volume and the icing looks glossy and stiff, about 5 to 6 minutes.
Transfer about 1/4 of the icing to a pastry bag with the #2 tip attachment. Add a teaspoon or two of water to the icing and mix. Transfer the remaining icing to a second pastry bag with a #3 tip attachment. Outline the cookies and immediately fill the cookies, spreading the icing out using a toothpick. Top with sanding sugar, luster pears or any sort of decoration that you'd like. In my case, I allowed the icing to dry completely and then dotted each cloud with polka dots. After the polka dots dried, I brushed gold dust (that I made into a paste by mixing it with a drop of vodka) onto each of the dots. Allow the cookies to dry completely before packaging.