Preheat oven to 375°F (190°C) and adjust rack to the middle position.

In a small bowl, whisk together egg yolk, heavy cream, and vanilla extract until well combined; set aside.

In a standing mixer, cream together softened butter, sugar, and salt on medium-high speed until light and fluffy (3-4 minutes).

Scrape down the bowl, then with the mixer running at medium speed, add the yolk-cream mixture and beat until just incorporated (about 30 seconds).

Scrape down the bowl again, then with the mixer running at low speed, gradually add in the flour until combined.

Scrape down the bowl and give a final stir with a rubber spatula to ensure no flour pockets remain.

If using a cookie press, fill it according to the manufacturer's instructions.

If using a pastry bag, fill the bag about halfway, then twist tightly to squeeze out any air.

Press or pipe cookies onto ungreased baking sheets, spacing them about 1 1/2 inches apart.

Bake one sheet at a time until the cookies are light golden brown (10-12 minutes), rotating the baking sheet halfway through baking.

Cool the cookies on the baking sheet until just warm (10-15 minutes).

Transfer cookies to a wire rack and cool completely.

For Lemon Spritz Cookies, add 1 teaspoon of lemon juice to the yolk-cream mixture and 1 teaspoon of finely grated lemon zest to the butter mixture.

For Almond Spritz Cookies, grind 1/2 cup of sliced almonds and 2 tablespoons of flour in a food processor until powdery. Combine with the remaining flour. Substitute 3/4 teaspoon of almond extract for the vanilla.