Prepare the Cookie Dough:

1. Cream the Butter and Sugar: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
2. Add the Egg and Vanilla: Beat in the egg and vanilla extract until the mixture is smooth and well combined.
3. Combine the Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, salt, cocoa powder, and cornstarch.
4. Mix the Dry Ingredients into the Wet Ingredients: Gradually add the dry ingredients into the wet mixture, mixing until just combined. The dough should be soft and slightly sticky.
5. Chill the Dough (Optional): For easier handling, refrigerate the dough for 20-30 minutes before shaping the cookies.

Shape the Cookies:

6. Preheat the Oven: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
7. Form Cookie Balls: Scoop tablespoon-sized portions of dough and roll them into balls. Place the balls on the prepared baking sheet, spacing them about 2 inches apart.
8. Create the Thumbprint: Use your thumb or the back of a spoon to gently press down in the center of each dough ball, creating an indentation for the filling.

Bake the Cookies:

9. Bake the Cookies: Bake the cookies for 10-12 minutes or until they are firm around the edges but still soft in the center.
10. Cool the Cookies: Let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.

Prepare the Filling:

11. Melt the Chocolate: In a small saucepan over low heat, melt the chocolate chips with the heavy cream, stirring until smooth and glossy.
12. Fill the Thumbprints: Once the cookies are completely cool, spoon a small amount of strawberry jam into each indentation, followed by a spoonful of the melted chocolate mixture.