Preheat your oven to 350°F. While it heats, place the slivered almonds on a baking sheet and toast them for 8-9 minutes until they turn light golden brown—you'll notice the aroma will become nutty and fragrant. Remove and set aside to cool completely. Meanwhile, melt one stick of butter in a small saucepan over medium-low heat, stirring occasionally. Once it turns golden brown with a nutty aroma (about 8-10 minutes), remove from heat and let cool for 10 minutes. The brown butter adds incredible depth to these cookies, so don't rush this step.

Once the toasted almonds are cool, pulse them in a food processor with the granulated sugar until the almonds are finely ground into a flour-like texture with no large pieces remaining—this creates the signature almond flavor throughout the dough. In a mixer bowl, beat the remaining stick of softened butter until light and fluffy (about 2-3 minutes). Add the cooled brown butter and the almond-sugar mixture, mixing well to combine. Add the eggs one at a time, beating after each addition, then add the vanilla extract. I like to make sure each egg is fully incorporated before adding the next—this helps create a more tender cookie.

In a separate bowl, whisk together the flour, salt, and baking powder. Pour the dry mixture into the wet ingredients from Step 2 and fold everything together with a spatula until just combined—the dough should come together but you don't want to overmix, which would make the cookies tough. Divide the dough into two equal discs, wrap each in plastic wrap, and refrigerate for 45 minutes. Chilling the dough is essential because it prevents spreading and helps the delicate almond flavor shine through.

Remove one dough disc from the fridge. Lightly flour your work surface, the dough, and your rolling pin, then roll the dough to just under 1/4 inch thick. Using a 2.5-inch round cutter, cut out 16 circles and place them on parchment-lined baking sheets. Bake at 350°F for 11 minutes until the edges are lightly golden, then remove and cool completely on the baking sheet. These will be your cookie bottoms that hold the Nutella filling.

Remove the second dough disc from the fridge and repeat the rolling and cutting process: roll to just under 1/4 inch thick and cut 16 circles with the 2.5-inch cutter. This time, use a smaller cutter (about 1 to 1.5 inches) to cut out a decorative shape from the center of each circle—this creates the signature Linzer cookie window that shows off the Nutella filling underneath. Bake these tops at 350°F for 8-9 minutes (slightly less time since they're thinner with the cutout), until lightly golden, then cool completely on the baking sheet.

While the cooled cookie tops are still on the baking sheet, dust them generously with powdered sugar, which adds a classic finishing touch. Spread about 2 teaspoons of Nutella onto the flat side of each cooled bottom cookie, then gently press a powdered-sugar-dusted top cookie onto it, being careful not to squeeze out the filling. The cookies are now ready to enjoy—they'll stay fresh in an airtight container for up to 5 days.