Preparation:

1. Bring all the butter (200g) to room temperature. Line a baking sheet with parchment paper (or use a Silpat).
2. Preheat the oven to 170°C (about 340°F) about 10 minutes before using.

Rum Raisins:

1. For oil-coated raisins, place them in a strainer and pour hot water over them to remove the oil. Pat dry with paper towels.
2. In a sterilized jar (or a resealable bag), place the raisins and pour in enough rum to cover them completely. Add a little extra rum as the raisins will absorb it over time. If the raisins are not fully submerged after a few days, add more rum.
3. You can use the rum raisins the next day, but it's recommended to wait about a week for the raisins to absorb the rum fully.

Cookie Dough:

1. In a bowl, soften the room-temperature butter and mix in powdered sugar until well combined.
2. Add the egg yolk and mix.
3. Sift in the cake flour and gently fold it in with a rubber spatula.
4. Flatten and shape the dough, wrap it in plastic wrap, and let it rest in the refrigerator for at least 1 hour.

White Chocolate Buttercream:

1. Melt the white chocolate in the microwave or over a double boiler.
2. In a bowl, soften the butter and mix until smooth. Gradually add the melted white chocolate and mix well.
3. Add the rum raisins and mix.

Spreading the Cream:

1. Line a shallow dish with plastic wrap and pour in the buttercream, spreading it to about 1cm thickness.
2. Chill in the refrigerator until firm, for about 1 hour.

Rolling and Cutting the Dough:

1. Prepare two large sheets of parchment paper and place the dough between them. Roll it out to about 3mm thickness using a rolling pin while tapping gently.
2. Chill the rolled-out dough in the refrigerator (or freezer) until it's firm enough for easy cutting.
3. Peel off the parchment paper from the top and use cookie cutters to cut out the dough. In this case, we used a 6cm x 6cm square shape.

Baking:

1. Arrange the dough pieces on the baking sheet and bake in a 170°C (about 340°F) oven for about 15 minutes, or until they turn a light golden brown.

Assembling the Sandwich Cookies:

1. Once the buttercream has hardened in the refrigerator, use a cookie cutter to carve out buttercream pieces of the same size as the cookies. (As the buttercream softens quickly, you can place it in the refrigerator or freezer as needed to make handling easier.)
2. Sandwich the buttercream pieces between two sablé cookies and voilà! You have your delicious Rum Raisin Buttercream Sandwich Cookies!