Step 1

Place 2 1/2 sticks unsalted vegan butter in the bowl of a stand mixer (or large bowl if using an electric hand mixer). Let sit at room temperature until softened.

Step 2

Arrange a rack in the middle of the oven and heat the oven to 350ºF. Coat a 9-inch square baking pan with cooking spray or softened vegan butter and line the bottom and 2 sides of the pan with parchment paper to form a sling.

Step 3

Grind 1 1/3 cups old-fashioned oats in a blender or spice grinder into a coarse meal (or finer if your blender is high-speed). Transfer to a medium bowl, add 2 cups all-purpose flour and 1 teaspoon kosher salt, and whisk to combine.

Step 4

Add 1 cup granulated sugar, and 1 teaspoon vanilla extract to the bowl of butter. Beat with the paddle attachment on the lowest speed just until combined, about 2 minutes. Don’t overmix. Add the flour mixture and mix on the lowest speed until just combined, 30 to 40 seconds.

Step 5

Scrape the dough into the pan. Using your hands, press the dough evenly into the pan and smooth the top. Cut the dough in the pan with a paring knife to make 18 rectangular bars: Make 2 parallel horizontal cuts that divide the slab into thirds, then make 5 parallel vertical cuts; cut all the way through the dough. Poke each bar with the blunt end of a skewer 6 times, poking about halfway down.

Step 6

Bake for 25 minutes. Remove the baking pan from the oven. Recut the lines and re-poke the holes. Rotate the baking pan and return to the oven. Bake until the shortbread is lightly browned, dry to the touch, and is pulling away from the sides of the pan, about 15 minutes more.

Step 7

Remove the pan from the oven and immediately cut the shortbreads along the scored lines, cutting all the way down to the bottom of the pan. Let cool completely in the pan, about 1 1/2 hours. Remove the bars from the pan, trim the edges with a serrated knife if desired, and place them on a wire rack fitted over a baking sheet.

Step 8

Place 1/4 cup unsweetened oat milk in a small microwave-safe bowl and microwave until hot, about 30 seconds. Add 2 teaspoons espresso powder and whisk with a fork until dissolved.

Step 9

Place 1 cup powdered sugar in a medium bowl. Add 1/2 teaspoon vanilla extract and whisk in the espresso-oat milk mixture a teaspoon at a time until combined and smooth, adding a drop or two more espresso-oat milk if the glaze is too thick, or more sugar if it is too thin. (You may not use all the espresso-oat milk.)

Step 10

Dip a fork or spoon into the mixture and gently drizzle back and forth over the bars a few inches above the bars, letting it fall over the bars in a decorative pattern. (Alternatively, dip half of each bar in the drizzle and return to the rack.) Let the drizzle set before serving, about 15 minutes.