Line your jumbo muffin tin with liners and set aside. Melt the butter and prepare to measure the rest of your ingredients as it cools.
In a small bowl, make the streusel. Combine flour, white sugar, and cold cut up butter. Using your hands, mix it together until combined into a crumbly, streusel mixture. Set aside while you make the muffin batter.
3/4 cups all-purpose flour, 1/3 cup white sugar, 6 tbsp butter
In a large bowl, whisk melted and cooled butter, white sugar, egg, egg yolk, buttermilk, oil, and vanilla. Whisk until combined.
1/2 cup butter, 2 tbsp oil, 1 egg, 1 egg yolk, 1 cup buttermilk*, 1 tbsp vanilla, 1 cup white sugar
Add in flour, salt, cornstarch, baking soda, and baking powder. Using a spatula, gently fold until just combined. It's important not to over mix with muffins.
2 1/2 cups all-purpose flour, 3 tsp baking powder, 1 tsp baking soda, 1/2 tsp salt, 1 tsp cornstarch
Toss in the mini chocolate chips and fold a few more times.
1 cup mini chocolate chips
Evenly divide your muffin batter between your liners. Make sure they are about 3/4s of the way full max, no more or else they will overflow. This recipe made 8 jumbo muffins for me, but depending on the size of your jumbo pan it can make more or less.
Bake the muffins at 425°F for 5 minutes, then turn down to 375°F and bake for an additional 15-20 minutes. The muffins are ready when they bounce back at touch or a toothpick inserted comes out clean.
These muffins are best eaten fresh. They stay fresh covered at room temperature for a few days, or in the refrigerator in an airtight container for up to 1 week!