To make the puff pastry: In a large mixing bowl, whisk together the flour, salt, and baking powder.  , Add the butter to the dry ingredients. Using your fingertips, a bowl scraper, or a pastry blender, work in the butter until it forms a coarse/crumbly mixture. Leave most of the butter in large, pea-sized pieces. , Use a flexible spatula to stir in the sour cream; the dough will be dry and won't be cohesive. Turn it out onto a large piece of parchment. Fold the parchment over the dough to bring it together. Flatten the dough into an 8" x 10" rectangle. , Cover the dough (sealing it in an all-purpose bread bag works well) and refrigerate it for 30 minutes to allow the dough to hydrate and keep the butter cool.  , Remove the dough from the refrigerator and uncover. Dust both sides with flour. Starting with a shorter end, fold one third into the center, then the opposite third over the first, like you were folding a business letter; this is called a letter fold.  , Flip the dough over, turn it 90° (a quarter turn) on your work surface, and roll it into an 8" x 10" rectangle. Perform another letter fold. , Wrap the puff pastry and return it to the refrigerator to chill for at least 30 minutes (or up to 2 days) before using. , To make the filling: Cut the guava paste into 8 even portions (about 15g or 1/2 ounce each).  , In a large bowl or the bowl of a stand mixer, beat the cream cheese, confectioners’ sugar, and butter until smooth.   , To assemble the guava pastries: Divide the puff pastry in half (about 290g per piece). Return one half, wrapped, to the refrigerator until ready to use. , Dust a clean work surface with flour. Working with one portion of puff pastry at a time, roll it into an 8" x 15" rectangle, then trim the edges until you end up with a 7" x 14" rectangle. (Save the scraps and bake them separately for a baker’s snack!) , Cut the rolled-out rectangle into eight 3 1/2" squares. Place the squares on a baking sheet lined with parchment and refrigerate while you roll and cut the remaining puff pastry in the same way (steps 11 and 12). , Remove the chilled dough from the refrigerator and place a generous tablespoon (20g) of cream cheese filling in the center of one square of dough; a tablespoon cookie scoop works well here. Add 1 portion of guava paste on top of the cream cheese. Brush the edges of the pastry with the egg wash, then place another square of pastry on top and seal the edges using your fingertips or the tines of a fork. Repeat with the remaining dough and filling until all of the guava pastries have been filled.  , Chill the assembled guava pastries for 20 to 30 minutes in the refrigerator before baking. While the pastries are chilling, preheat the oven to 375°F. , When ready to bake, brush the tops of each pastry with egg wash, score the tops using a paring knife, and sprinkle with sparking sugar.  , To bake the guava pastries: Bake the pastries for 25 to 30 minutes, until golden brown. Serve warm or at room temperature. , Storage information: Store any leftover guava pastries, covered, at room temperature for up to 3 days.