Prepare your workspace and ingredients. Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. Allow butter to come to room temperature for easier mixing.
Combine the dry ingredients. In a medium bowl, whisk together the flour, cocoa powder, salt, and baking powder until well blended and no cocoa lumps remain.
Cream butter and sugar. In a large bowl, beat the softened butter and sugar with an electric mixer on medium speed until light and fluffy, about 2-3 minutes. Don’t rush this step as proper creaming creates the perfect texture.
Add wet ingredients to the butter mixture. Beat in the egg yolk, milk, vanilla extract, and almond extract (if using) until fully incorporated, scraping down the sides of the bowl as needed.
Incorporate the dry ingredients. Gradually add the flour-cocoa mixture to the wet ingredients on low speed, mixing just until combined. Overmixing will result in tough cookies.
Shape the cookies. Roll the dough into 1-inch balls (about 1 tablespoon each) and place 2 inches apart on the prepared baking sheets.
Create thumbprints in each cookie. Using your thumb or the back of a ½ teaspoon measuring spoon, make an indentation in the center of each ball, pressing down about halfway through the cookie.
Fill with jam. Spoon about ½ teaspoon of strawberry jam into each indentation, being careful not to overfill.
Bake until set. Place in the preheated oven and bake for 10-12 minutes, until the cookies are set but still slightly soft (they’ll firm up as they cool).
Cool properly. Allow cookies to cool on the baking sheets for 5 minutes before transferring to wire racks to cool completely.
Finish with a touch of elegance. If desired, lightly dust the cooled cookies with powdered sugar, avoiding the jam centers for a beautiful contrast.