A little cream cheese in the dough makes these pretty, swirly cookies especially tender.
In a large bowl, using an electric mixer (or in a stand mixer fitted with the paddle attachment), beat the 2 sticks of butter, cream cheese, 3 tbsp. granulated sugar, 1 tbsp. brown sugar, the vanilla, and salt on medium speed until smooth, about 2 minutes. Add the flour. Beat on low speed until just blended.
Pat the dough into an 8-inch square. Wrap in plastic and place on a baking sheet. Refrigerate until firm, about 1 hour.
Arrange racks in the top and bottom thirds of the oven; preheat to 350°. Line 2 baking sheets with parchment paper. In a spice mill, pulse the poppy seeds and the remaining 3 tbsp. brown sugar just enough to break up the seeds.
Cut the dough into 4 pieces. Working with 1 piece at a time (keep the other pieces covered and chilled), roll out the dough into a 4-by-12-inch rectangle. Brush with some melted butter. (This is my kids’ favorite job.) Sprinkle the dough with about a quarter (2 to 3 tbsp.) of the poppy seeds; lightly press the seeds with your fingers (another favorite kid job!). Starting with 1 long side, roll the dough into a 12-inch log. Place, seam-side down, on a work surface. Cut the log into 1-inch pieces. Repeat with the remaining dough, some more butter, and the remaining poppy seeds. Arrange about 24 rugelach, spaced evenly apart, on each baking sheet.
Brush the tops of the rugelach lightly with the remaining melted butter. Sprinkle evenly with the remaining 2 tbsp. granulated sugar. Place in the freezer for 5 minutes or in the refrigerator for 10 minutes so the dough firms up before baking. (This is especially important if the dough has gotten soft or sticky while rolling and filling.)
Bake until the rugelach are lightly golden brown, 22 to 25 minutes. Let cool on the baking sheets for 5 minutes. Transfer the cookies to wire racks to cool completely.