Whisk the flour and salt together in a small bowl.
In a stand mixer, beat the butter and sugar together on medium speed until they become fluffy, about 2 minutes. Scrape down the bowl.
Add the egg, egg yolk, vanilla extract, and beat on low until fully incorporated, about 2 more minutes. Scrape down the bowl as necessary to enable incorporation.
Dump in the flour and salt, zest, cranberries, and poppy seeds. Beat at low speed until not quite combined. Stop the mixer and use a spatula to fold everything together until incorporated.
Divide the dough in half. Place each half on a separate sheet of wax or parchment paper. Gently roll or pat each into a 1-inch diameter log (the wax paper will help you do this). Wrap with plastic wrap and store in the refrigerator until fully chilled, at least 3 hours or overnight.
Preheat the oven to 350 F, ensuring one oven rack is in the middle and the other is in the upper third. Line your cookie sheet pan(s) with parchment paper.
Remove the dough from the refrigerator and it's protective plastic wrap, then slice the dough into ½-inch-thick rounds*. Place them 1-inch apart on the cookie sheet(s).
Bake for 11-15 minutes, until barely golden at the edges. Remove from the oven and cool on the cookie sheet on a wire rack for at least 10 minutes.