Heat oven to 350F. Line a large cookie sheet with parchment paper.

In a large bowl, whisk flour, cocoa powder, salt and baking powder until well combined. In a separate large bowl with an electric hand mixer or in the bowl of a stand mixer with the paddle attachment, beat butter and sugar on medium-high speed until light and fluffy, about 2-3 minutes. Stir in eggs on low speed one at a time, then stir in vanilla until just combined. Gradually stir in flour mixture on low speed until just combined. Cover and refrigerate dough at least 1 hour.

On a well-floured surface, roll out chilled dough to 1/4-inch thick. Use a cookie cutter (I used a 2-inch linzer cookie cutter) to cut out cookie rounds; transfer to prepared cookie sheet, spaced 1 inch apart (you'll have to bake in batches). Re-roll dough once, if needed, to cut more rounds. Bake 8-10 minutes until set and puffed. Cool 1 minute on baking sheet, then transfer to a cooling rack to cool completely.

Pipe or spoon approximately 1 tablespoon dulce de leche on the bottoms of half of the cookies. Top with remaining half of cookies. Store covered in an airtight container up to 5 days.