• Set your oven to 350°F (175°C) so it has plenty of time to heat. Generously coat a 24-cup mini muffin pan with baking spray, making sure every cavity is well covered. A baking spray that contains flour gives the cleanest release, especially with these rich, fudgy brownie bites.
• If your muffin pan tends to hold onto baked goods, line each cup with two narrow strips of parchment paper placed in a crisscross pattern after spraying. This makes lifting the brownie bites out much easier.
• Place the butter in a medium saucepan over medium heat and let it melt completely. Remove the pan from the heat, then stir in the neutral oil, granulated sugar, brown sugar, cocoa powder, and chopped dark chocolate.
• Return the saucepan to low to medium heat and stir constantly until the chocolate melts completely and the mixture becomes smooth and glossy with no dry cocoa remaining. Keep the mixture moving to prevent scorching. A silicone spatula works beautifully, though a whisk is equally effective.
• Take the saucepan off the heat and let the chocolate mixture cool for 5 minutes, allowing it to become warm rather than hot.
• Add the eggs and whisk vigorously for 1 to 2 minutes. At first the mixture may look separated, but keep whisking and it will quickly turn smooth, thick, and shiny. This step also helps create the signature crackly brownie top.
• Add the flour and salt, then gently fold everything together with a spatula just until no streaks of flour remain. Avoid overmixing to keep the brownie bites soft and fudgy.
• Using a small cookie scoop or measuring spoon, divide the batter evenly among the prepared muffin cups, filling each about two-thirds to three-quarters full. The batter should be enough to fill all 24 cavities.
• Bake for 15 minutes, checking around the 12-minute mark if your oven bakes quickly. The brownie bites are ready when the tops look glossy with delicate cracks and the centers are just set. Be careful not to overbake.
• Let the brownie bites cool in the pan for 15 minutes. To remove them, gently twist each brownie inside its cup before lifting it out with a butter knife or small offset spatula. Twisting first helps release the edges cleanly and prevents pieces from sticking to the pan.

Filling

• Allow the brownie bites to cool in the pan for about 10 minutes after baking. Using the back of a 1 teaspoon measuring spoon or another small rounded spoon, gently press into the center of each brownie to create a shallow well. Make the indentation as deep as you like, but avoid pressing all the way through the bottom.
• Leave the brownie bites in the pan for another 5 minutes, then gently twist and lift them out. Transfer them to a wire rack and let them cool completely before filling each center with your favorite fillings.