Start by creaming butter and sugars until light and fluffy. Beat in eggs and vanilla, then gradually mix in dry ingredients and oats. Fold in raisins if using. Chill the dough slightly for easier handling.
Scoop dough into uniform rounds on a parchment-lined baking sheet. Bake at 350°F (175°C) for 12-15 minutes until edges are lightly golden. Cool completely on a wire rack to ensure the filling doesn’t melt.
Beat butter and shortening together until smooth. Gradually add powdered sugar, vanilla, and milk until creamy and spreadable. Adjust milk to reach desired consistency for easy sandwiching.
Spread a generous layer of vanilla cream on one cookie, then top with a second cookie. Press gently to secure. Repeat until all cookies are paired.
For best results, refrigerate the cream pies for 30 minutes before serving. This allows the filling to set slightly for a perfect bite.