Preheat oven to 325 degrees. Butter twelve 3 1/2-inch brioche molds or a standard 12-cup muffin tin. Dust with flour; tap out excess. Whisk flour and salt in a medium bowl. Put butter and granulated sugar into the bowl of an electric mixer fitted with the paddle attachment; cream on medium speed until light and fluffy. Mix in eggs and yolks, 1 at a time. Add vanilla and 1 1/2 teaspoons water; mix until combined. Reduce speed to low. Add flour mixture; mix until just combined, scraping down sides as needed. Spoon 1/3 cup batter into each mold.

Beat cream cheese and confectioners' sugar in the clean bowl of the electric mixer on low speed until creamy and combined. Spoon 1 tablespoon mixture onto batter in each mold; sprinkle with poppy seeds. Bake until a cake tester comes out clean, about 25 minutes. Let cool 5 minutes; transfer cakes to a wire rack to cool completely. Serve dusted with confectioners' sugar.