Preheat the oven to 350℉/176℃. Grease the muffin pan with oil very well.
Brown the butter. Melt the butter in a medium sized pan over medium heat. Once it melts, it will crackle, then become foamy – continue mixing the butter while this happens to prevent burning. The butter will go from melted to golden brown quickly, and you’ll know it’s almost done when the aroma of browned butter arrives. The whole process should take about 5 minutes! Once golden, remove from heat and transfer to a glass bowl. Let the butter come back down to room temperature (minimum 15-30 minutes) while you prep on the rest of the recipe.
3/4 cups + 1 tbsp unsalted butter
In a medium-sized bowl, add in flour, baking soda, baking powder, cornstarch, and salt. Mix together and set aside.
1 3/4 cups all-purpose flour, 1/2 tsp baking soda, 1/4 tsp baking powder, 2 tsps cornstarch, 1/2 tsp salt
In a large bowl, add in the brown butter, white sugar, and dark brown sugar. Whisk together for 2-3 minutes.
1/2 cup dark brown sugar, 3/4 cups + 1 tbsp unsalted butter, 1/3 cup white sugar
Add in the eggs and vanilla. Continue to whisk for another 1-2 minutes, until lighter in colour and air bubbles have formed.
2 large eggs, 1 tsp vanilla
Toss in the dry ingredients. Using a spatula, mix until almost completely combined.
Add in the semi-sweet chocolate chips and toffee bits. Mix until evenly distributed and now no flour streaks remain, make sure not to over-mix!
1/2 cup toffee bits, 3/4 cups semi-sweet chocolate chips
Scoop approximately 2 tbsp per muffin tin. I used a cookie scooper for even scooping and recommend you do the same! You can also scoop them with spoons. Add extra chocolate chips and toffee bits on top if desired (optional, but recommended).
Bake the cookie cups for 11-14 minutes. Mine took exactly 13 minutes. If you bake them for a shorter amount of time they will be extra gooey. If you bake them longer, then they will be more crisp. They are delicious either way!
Remove the cups from the oven and allow them to come back to room temperature. Then gently run a knife around each one and pop them out of the pan.
They are best served fresh or next day. I personally love eating them warm right out of the oven. You can store them in an airtight container on your counter for 3 days, or in the fridge for up to 1 week.