Combine butter and sugars in a large bowl (or in the bowl of a stand mixer) and use an electric mixer to beat until creamy.
Add egg yolk and stir well.
Add salt, vanilla extract, and almond extract (if using). Beat until well-combined.
With mixer on low-speed, gradually add flour until completely combined. Dough may be crumbly, if needed use a spatula to scrape the bottom and sides of the bowl and work the flour into the dough.
Add nonpareils (if using) and fold into dough (try not to over-work it at this point as nonpareils may bleed).
Divide dough into 2 parts and transfer each part onto a sheet of wax paper. Work dough into a 2" wide log shape (I like to use the wax paper to help do this, see video for reference) and transfer to the refrigerator to chill for at least 2-3 hours.*
Once dough has chilled, preheat oven to 350ºF (175ºC) and line a cookie sheet with parchment paper.
Remove one cookie dough log from the refrigerator and use a sharp knife to cut into 1/4" slices.
Place slices on prepared cookie sheet about 1" apart and bake on 350ºF (175ºC) for 10-12 minutes or until edges are just beginning to turn golden brown.
Allow to cool on baking sheet before serving. If desired, dip cooled cookies in chocolate and top with additional sprinkles.
*While you can place the dough directly in the refrigerator, since it is still soft at this point it tends to get a flat bottom, giving you less than round cookies when you go to slice them. To prevent this, I like to cut a paper towel roll in half and then cut a slit lengthwise and wrap my cookies in the paper towel roll (see video), it keeps the cookie logs completely round!See post for instructions on dipping your slice and bake cookies in chocolate!