Set the paddle attachment and beat the butter until fluffy and soft. Add vanilla extract and mix.
1 cup Butter, 2 teaspoon Vanilla extract
Mix all the dry ingredients, then add to the butter.
1½ cup All-purpose flour, 1¼ cup Powdered sugar, ⅓ cup cocoa powder, 1 tablespoon Cornstarch, ¼ teaspoon Salt
Beat the dough for about a minute. It should have the consistency of play dough. Wrap the dough in cling wrap and chill for 2 hours.
Preheat the oven to 300°F, and line two baking sheets with parchment paper. Roll out the dough to ¼ inch thickness. Use your heart-shaped cookie cutter to cut the cookies. Place them on the lined baking sheets and bake for 13-15 minutes. Cookies are done when the edges look set.
Let them cool down before filling them.
Assembly
Arrange all the cookies with no holes in the center, on the working table. Place about one teaspoon of Raspberry or Strawberry jam in the center of each cookie. Place the matching cookie with a hole, on the jam and gently press down.
½ cup Raspberry jam
I used milk chocolate for decorating because it contrasts with the cookies' colours. You can use dark chocolate if you like. Melt the chocolate and pour it into a piping bag. Cut the tip of the bag into a small hole and start running the melted chocolate over the cookies.