Beat remaining 1/4 cup butter and 1/2 cup granulated sugar in a large bowl at medium speed with an electric mixer until blended. Add egg yolks, 1 at a time, beating just until blended after each addition. Add milk, sour cream, and vanilla, beating just until blended. Combine baking mix and cinnamon. Add cinnamon mixture to milk mixture, beating just until blended.
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