Step 1: In a large mixing bowl, combine the all-purpose flour, 1/2 cup granulated sugar, softened unsalted butter, egg, vanilla extract, and salt. Mix using a stand mixer with a paddle attachment or a hand mixer on low speed, just until a cohesive dough forms. Be careful not to overmix.
Step 2: Roll the dough into uniform balls, about 1 inch in diameter. Place them on a baking sheet lined with parchment paper. Use your thumb or the back of a small measuring spoon to create a deep indentation in the center of each cookie ball.
Step 3: In a small saucepan, combine the raspberries (if using fresh, crush them lightly), 1/4 cup granulated sugar, and lemon juice. Cook over medium heat, stirring occasionally, for about 5-7 minutes, or until the mixture thickens to a jam-like consistency. Remove from heat and let cool slightly.
Step 4: Spoon the cooled raspberry jam into the thumbprint of each cookie. Then, sprinkle the toasted sliced almonds evenly over the jam, followed by a light dusting of the remaining 1 tbsp granulated sugar.
Step 5: Bake in a preheated oven at 350°F (175°C) for 12-15 minutes, or until the edges of the cookies are lightly golden and the shortbread is set. Allow them to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
Step 6: If desired, dust the cooled cookies with powdered sugar and garnish with a fresh raspberry before serving. Enjoy your delicious homemade shortbread thumbprint cookies!