The Trials & Tribulations of Baking a Buttermilk Pie

A few days ago, I baked a buttermilk pie from this book

I think the end product came out very tasty; HOWEVER, boy did I have trouble with the filling.

The filling is made with a basic butter, sugar, flour, and buttermilk custard. I creamed the room temperature butter and sugar for about 5 minutes, added in 3 eggs VERY SLOWLY (really, very, very, very, slowly), and everything was going great. The SECOND I started to mix in the flour, my mixture curdled. AND I DON’T UNDERSTAND WHY!!! When I added the buttermilk, it was too late. My mixture curdled, hard. I can’t express my despair enough. All my ingredients were at room temperature, I mixed in my eggs one at a time, I followed all the tips I found online to not have my mixture curdle. And it still happened. I’ve seen other recipes use melted butter, and I’m wondering if I should try that.

Anyways, I baked my pie with the curdled mixture and it came out great anyways; the filling had a great texture. Some recipes even said that curdling is fine. But, I’ve seen videos on Youtube of people making this pie and their mixture doesn’t curdle, and I want to know why.

So, if anyone has any tips, please let me know!