1 cup unsalted butter, softened to room temperature
2/3 cup granulated sugar
1 teaspoon vanilla extract
1/2 teaspoon almond extract
2 cups Flour
1/2 cup blackberry jam
1 cup confectioners' sugar
1 Tablespoon + 1 tsp. lemon juice from fresh lemon
Make the cookies: Using a handheld or stand mixer with a paddle attachment, beat the butter on high speed until creamy, about 1 minute. Switch mixer to medium speed and add the the sugar, vanilla, and almond extracts. Scrape down the sides and the bottom of the bowl as needed. Turn the mixer off and pour the flour into the wet ingredients. Turn the mixer on low and slowly beat until a very soft dough is formed. Press the dough down to compact it and tightly cover with plastic wrap to chill until firm, at least 4 hours.
Preheat oven to 350* degrees. Line two baking sheets with parchment paper or silicone baking mats (silicone mats preferred to reduce spreading). Shape the cookie dough into balls. Mine were about 1 Tablespoon of dough per ball. Make sure they're nice and smooth. If you find that the balls of dough are sticky and/or have gotten a little soft after rolling- place the balls of dough back into the refrigerator to firm up. You absolutely DO NOT want soft dough. Make an indentation with your thumb into each ball. The dough may crack slightly when you press your thumb into it. Simply smooth it out with your fingers if you can. Otherwise, it's perfectly fine to have a few cracks. Fill each with a scant 1/2 teaspoon of jam. (Or however much it can hold.)
Bake the shortbread thumbprint cookies for 14-15 minutes, or until very lightly browned on the edges. The cookies will puff up and spread slightly. Do not over bake. In fact, I only baked mine for 13-14 minutes. I prefer them a little soft. Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack. Allow to cool for at least 30 minutes before glazing.
Make the glaze: stir together glaze ingredients until smooth. Add more liquid to thin out or add more confectioners' sugar to thicken to your desired consistency. Drizzle over cooled cookies. Glaze will set within a couple hours.
Store cookies covered at room temperature for 3 days or in the refrigerator for 6 days. Shortbread cookie dough may be frozen up to 2 months; baked cookies (without glaze) may be frozen up to 2-3 months.