For the Cupcakes:

1. In a medium bowl, combine chopped chocolate and cocoa powder.

2. In a measuring cup, mix hot water and espresso powder, then pour over the chocolate mixture. Whisk until smooth. Cover with plastic wrap and let it cool.

3. Preheat oven to 350°F (175°C). Line two 12-cup muffin tins with cupcake liners.

4. In a small bowl, whisk together flour, sugar, salt, and baking soda.

5. Whisk oil, eggs, vinegar, and vanilla into the cooled chocolate mixture until smooth.

6. Gradually mix in the dry ingredients until fully combined.

7. Divide the batter evenly into the muffin cups, filling each 3/4 full.

8. Bake for 17-19 minutes, or until cupcakes are set and firm when lightly pressed.

9. Let the cupcakes cool completely, then use an apple corer or paring knife to remove a small section from the center of each cupcake. Fill with 1 heaping tablespoon of Nutella.

For the Strawberry Buttercream:

1. In a stand mixer, beat butter until light and fluffy.

2. Add vanilla extract and strawberry jam, mixing well.

3. Gradually add powdered sugar, one cup at a time, beating on medium speed until smooth.

4. Transfer the buttercream to a piping bag fitted with a decorative tip and frost the cooled cupcakes.