Prepare your workspace and ingredients. Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper. Make sure your butter is properly softened but not melting.
Cream butter and sugar until light and fluffy. Using an electric mixer, beat the butter and granulated sugar together on medium speed for about 3 minutes until the mixture becomes pale and creamy.
Add the almond extract and mix until well combined. The almond extract provides the signature flavor, so be precise with the measurement.
Gradually add the flour to the butter mixture, mixing on low speed just until the dough comes together. Overmixing will develop gluten and make your shortbread tough, so stop as soon as the flour is incorporated.
Chill the dough for 15 minutes in the refrigerator. This makes the butter firm up slightly, which helps the cookies maintain their shape during baking.
Shape the cookies by rolling the dough into 1-inch balls and placing them 2 inches apart on the prepared baking sheets.
Create thumbprints in each cookie by pressing your thumb or the back of a small measuring spoon into the center of each ball, making a depression about 1/2 inch deep.
Fill each thumbprint with approximately 1/4 teaspoon of raspberry jam. Avoid overfilling, as the jam can bubble over during baking.
If using sliced almonds, sprinkle them around the edges of the cookies, gently pressing to adhere.
Bake for 14-18 minutes until the edges are just barely turning golden. The cookies should remain pale on top.
Allow cookies to cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.
If making the optional glaze, whisk together the powdered sugar, milk, and almond extract until smooth. Drizzle over cooled cookies and allow to set before serving.