Melt the butter: In a microwave-safe bowl or small saucepan, melt the unsalted butter. Allow it to cool for at least 10-15 minutes until it's no longer hot, but still liquid.

Combine dry ingredients: In a medium bowl, whisk together the all-purpose flour, baking soda, baking powder, cornstarch, and salt. Set aside.

Cream butter and sugars: In a large mixing bowl, combine the slightly cooled melted butter with the packed light brown sugar and granulated sugar. Using an electric mixer on medium speed, beat for 2-3 minutes until light in color and fluffy. Scrape down the sides of the bowl as needed.

Add wet ingredients: Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until just combined.

Combine wet and dry: Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix; stop as soon as no streaks of flour remain.

Fold in chocolate: Gently fold in the semi-sweet chocolate chips and chopped dark chocolate bar using a rubber spatula.

Chill the dough: Cover the bowl with plastic wrap and refrigerate the cookie dough for at least 1 hour, or up to 24 hours. Chilling is crucial for thick cookies and prevents excessive spreading.

Preheat oven: Once the dough has chilled, preheat your oven to 375°F (190°C). Line two large baking sheets with parchment paper.

Scoop dough: Using a 3-tablespoon (or large) cookie scoop, portion out the chilled dough onto the prepared baking sheets, leaving about 2 inches between each cookie. For an extra "molten" look, you can press a few extra chocolate chips or chunks into the top of each dough ball.

Bake: Bake one sheet at a time for 10-12 minutes, or until the edges are golden brown and set, but the centers still look very soft and slightly underbaked. The cookies will continue to set as they cool.

Cool: Remove the baking sheets from the oven and let the cookies cool on the sheet for 5-10 minutes. This allows them to firm up without breaking. Then, carefully transfer them to a wire cooling rack to cool completely.