These easy icebox slice-and-bake cookies start with one buttery vanilla dough that you can customize with your favorite mix-ins. Shape the dough into logs, chill until firm, then slice and bake for fresh, tender-edged cookies whenever you like. The dough makes about 24 cookies and can be refrigerated for several days before baking.
In a large bowl, beat the softened butter, brown sugar, and granulated sugar with a handheld mixer or stand mixer fitted with a paddle attachment on medium-high speed until creamy and combined, about 3 minutes.
Beat in the egg and vanilla on high speed. Scrape down the sides and bottom of the bowl as needed, then continue mixing until fully combined.
Add the flour and salt and mix on low speed just until combined. Beat in your chosen flavor add-ins on low, stopping as soon as they are evenly distributed. The dough should be thick and slightly sticky; if it is too sticky to shape, mix in 1–2 tablespoons additional flour.
Turn the dough onto a lightly floured work surface and divide it in half. With floured hands, shape each half into an approximately 8-inch-long log, about 1 1/2 to 2 inches in diameter. Wrap each log tightly in plastic wrap and refrigerate for at least 4 hours, or up to 5 days.
When ready to bake, preheat the oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats.
If desired, spread coarse sugar on a plate and roll the chilled logs in it, pressing gently so the sugar adheres. Slice each log into 12 evenly thick cookies and arrange them about 2 inches apart on the prepared baking sheets.
Bake for 13–15 minutes, until the edges are very lightly browned. The cookies will remain fairly pale. Let them cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.
Once cool, dip or drizzle the cookies with melted chocolate and add toppings if desired. Store covered at room temperature for up to 1 week.