Preheat oven to 350°F. Line a baking sheet with parchment paper and set aside.
In the bowl of a stand mixer, beat together the butter and sugar until light and fluffy, about 5 minutes. Add the egg yolk and vanilla and blend until the yolk disappears.
In a separate medium bowl. whisk together the flour, cocoa powder and salt. Add the flour mixture to the butter and sugar. Mix on low until combined, then blend on medium for about a minute. Scrape down the sides and bottom of the bowl as needed with a rubber spatula. Blend another 30 seconds to make sure the all the ingredients are combined.
Using a small cookie scoop (about 1¼" wide - 2 teaspoons) roll the dough into tight balls and place about an inch apart on the prepared cookie sheet. To make the heart shape, use your smallest finger and make two indentations into the dough ball. Don't push your finger all the way through the dough as this will cause the cookie to spread too much and not hold it's shape. Fill the raw cookie with 1/4 teaspoon of raspberry jam.
Bake for 12-15 minutes or until the jam is bubbly and the cookies are puffed and set. Remove from the oven and allow the cookies to cool on the baking sheet for 3-5 minutes, then remove to a wire rack to cool.
Store the baked cookies in an airtight container at room temperature. Use wax paper between layers if necessary.