Preheat your oven to 400°F and line a 12-cup muffin pan with paper liners (or grease the cups well).
In a large bowl, whisk together the melted butter and granulated sugar for a full 2 minutes.
Add the eggs, vanilla extract, and cake batter extract. Whisk until smooth and combined.
Pour in the sour cream and whole milk and whisk until everything is silky and uniform.
Add the flour, baking powder, and salt right into the bowl, then gently FOLD them in with a spatula until just combined.
Fold in the ¾ cup of sprinkles, just enough to distribute them evenly throughout the batter.
Make the streusel by stirring together the melted butter, flour, sugar, and sprinkles in a separate bowl until you have big crumbly clumps.
Scoop the batter evenly into your 12 muffin cups, filling each almost all the way to the top.
Generously pile that streusel on every single muffin.
Bake at 400°F for 10 minutes, then WITHOUT opening the oven, drop the temperature to 350°F and bake for another 10 minutes.
Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack.
