Can I make “sour cream butter” by churning sour cream?

Sadly no. Believe me, I’ve tried it. Store bought sour cream simply has too little fat in it. Whereas heavy cream contains something on the order of 40% fat, the sour cream in the plastic tubs is a mere 18% fat, about 12% less than the minimum you need to whip (and then of course over-whip). Nope, for cultured butter and buttermilk, you need a real “soured” cream starting point.