In a small bowl, combine the all-purpose flour, melted butter, and granulated sugar until crumbly. Refrigerate the crumble while preparing the muffin batter.
In a large mixing bowl, whisk together the whole milk and granulated sugar.
Add the melted butter and whisk until well combined.
Add the eggs and whisk until smooth.
Add the sour cream and whisk until fully incorporated.
Stir in the vanilla extract.
In a separate bowl, whisk together the all-purpose flour, baking powder, and kosher salt.
Gradually add the dry ingredients to the wet ingredients and mix until just combined.
Fold in the semisweet chocolate chips.
Divide the batter evenly among the paper-lined muffin cups.
Sprinkle the chilled crumble evenly over each muffin.
Bake at 425°F (220°C) for 5 minutes, then reduce the oven temperature to 375°F (190°C) without opening the oven. Continue baking for 15–18 minutes, or until a toothpick inserted into the center comes out clean.
In a microwave-safe bowl or over a double boiler, melt the semisweet chocolate chips with the heavy cream. Stir until smooth, then transfer the melted chocolate to a piping bag and set aside.
Let the muffins cool in the pan for 5 minutes. Using the back of a spoon, gently make a small well in the center of each muffin.
Pipe the prepared melted chocolate into the center of each muffin. Garnish with chocolate chips, if desired.
The Chocolate Chip Muffins are ready to serve.
