Hi, this weekend I found a chocolate chip muffin recipe. it says mix 1/2 cup melted butter, 3/4 cup buttermilk, 2 large eggs and 1 TB vanilla Extract together until smooth (these are the wet ingredints). but when i mix them together, it was curdling up. i dump this first batch, i thought i did something wrong. so the 2nd time, i melted the butter (the butter is cool down) and then i slowly pour in the buttermilk and quickly whisk them, still curdling up. i would like to know is this normal? how do i make them don't curdle up? and how i do add buttermilk to the receipe?

thank you for helping!

the recipe:
2 cups AP four
3/4 cup sugar
2 Tsp baking powder
1/2 Tsp baking soda
1/2 Tsp salt

1/2 cup melted butter
3/4 cup butter milk
2 large eggs
1 TB vamilla extract