Steps:

• Preheat oven to 375 degrees F (190 degrees C). Line 2 baking sheets with parchment paper.
• Sift flour, 1 tablespoon sugar, and salt together into a bowl. Cut in chilled butter with a pastry blender or 2 knives until mixture resembles small peas. Add vanilla. Mix in cold water, 1 tablespoon at a time, until dough comes together and forms a ball.
• Divide dough into 2 pieces and wrap with plastic wrap; chill until firm, at least 1 hour.
• Beat softened butter, 1/4 cup sugar, and cinnamon together in a bowl to make filling.
• Roll out dough on a lightly floured work surface to 1/2-inch thickness. Cut dough into rectangles with a knife. Drop 1 tablespoon of filling onto half of the rectangles. Cover with remaining rectangles. Press edges with a fork to seal. Poke a few holes in the top of each pastry with a toothpick.
• Arrange pastries on the lined baking sheets.
• Bake in the preheated oven until golden brown, 25 to 30 minutes. Cool pastries on a wire rack, about 15 minutes.

Steps:

• Combine fruit filling ingredients and 3 tablespoons water in a medium pot over medium heat; stir. Gently mash berries with the back of a wooden spoon and simmer, stirring frequently, for 30 to 40 minutes, or until filling has thickened. Let cool. Cut chilled dough into 4 equal pieces. Roll 2 pieces into equal rectangles about 3- by- 8 inches. Fill a piping bag with Nutella and a second piping bag with marshmallow crème. Onto one of the rectangles, pipe rows of Nutella 1/2 inch apart. Pipe rows of marshmallow crème on top. To make the egg wash, whisk together egg and heavy cream. Using a pastry brush, carefully brush egg wash onto dough between the rows of filling. Gently lay the second dough rectangle over top and press around each mound of filling. (Tip: This is the same method used for homemade ravioli.) Cut 1- by- 1 1/2-inch rectangles with a pizza cutter (you should have 12 tarts) and press the back of a fork along the edges to seal. Place tarts on one of the prepared sheets and brush lightly with cream. Cut 3 tiny slits in the top of each tart. Bake for 12 to 15 minutes, or until golden brown. Place on wire racks to cool completely. Repeat the process with the remaining two slabs of dough and the cooled fruit filling to make 12 fruit-filled tarts. For the icing, in a medium mixing bowl or the bowl of a stand mixer, beat egg white on medium-high speed for 2 minutes, or until light and frothy. Add powdered sugar, 1/4 cup at a time, until mixture is thick and smooth. Add vanilla and beat for 1 minute more. Spread icing onto pastries with an offset spatula or your fingers. Top with sprinkles. For the pie dough, in a medium mixing bowl, sift together flour, baking powder, and salt. Add butter and cream cheese and cut into dry ingredients with a pastry knife or your fingers until fine crumbles are formed. Add buttermilk and, using your hands, gently work mixture until dough just comes together. Form dough into a disk, wrap with plastic wrap, and refrigerate until ready to use.