Preheat your oven to 350 degrees F (175 degrees C). Prepare a 9x13 inch metal cake pan by lining it with parchment paper or lightly greasing it with butter. This ensures easy removal of the brownies once baked.
In a large bowl, combine flour, cocoa powder, espresso powder, baking powder, and salt; set aside. This dry mixture is essential for ensuring a well-combined batter.
In a large saucepan, add the butter and melt it over very low heat, stirring every few minutes until it’s smooth. This gentle heating allows the butter to incorporate well with the sugar.
Add sugar to the melted butter and stir until the mixture becomes creamy and shiny. Remove the saucepan from heat and let it cool slightly.
After a few minutes, whisk in eggs, vanilla, and coffee into the butter-sugar mixture, mixing until smooth and well combined.
Now, add this warm butter and egg mixture to the dry flour mixture. Stir everything together until it’s smooth. Make sure there are no lumps!
Let the batter rest for about 5 minutes. This resting time allows the flour to absorb moisture, contributing to the fudgy texture.
Gently fold in the chocolate chunks, reserving a few for topping if you wish. This step ensures that every bite of the brownie will have a delightful chocolate surprise.
Pour the batter into the prepared cake pan and smooth out the top using a spatula for an even bake.
In a small microwave-safe bowl, place peanut butter and microwave it for 30 seconds. Stir it well and then spoon or drizzle it over the brownie batter. Use a knife to gently swirl the peanut butter into the batter, creating a marbled effect.
Sprinkle the toffee chips evenly over the top and finish with a sprinkle of sea salt. This adds a sweet and salty contrast.
Bake the brownies for 25 to 30 minutes. You’ll know they’re done when a tester inserted in the center comes out clean or with just a few crumbs attached. Remember, it's better to underbake slightly for that fudgy texture!
Cool the brownies in the pan on a wire rack at room temperature for about an hour. If you used parchment paper, lift them from the pan and cut into 30 squares. These brownies are incredibly rich, so small servings are perfect!
