Courtesy of https://theviewfromgreatisland.com/vanilla-on-vanilla-sandwich-cookies/

INGREDIENTS

Dry ingredients
1 1/2 cups all purpose flour
1/2 tsp salt
1/4 tsp baking powder

Wet ingredients
3/4 cup (1 1/2 sticks) unsalted butter, room temperature
1 1/4 cups sugar
1 tsp vanilla extract
2 large eggs

Vanilla filling
1 1/2 cups confectioner’s sugar
3 Tbsp unsalted butter, room temperature
1 1/2 tsp vanilla extract
milk or cream for thinning

Chocolate filling
1 1/4 cup confectioner’s sugar
1/4 cup unsweetened cocoa powder
3 Tbsp unsalted butter, room temperature
1/2 tsp vanilla extract
milk or cream for thinning

DIRECTIONS

Preheat oven to 375F
In a small bowl whisk together the dry ingredients. Set aside.
Cream the butter, then beat in the sugar and vanilla. Beat until creamy.
Beat in the eggs and blend well. Then add the dry ingredients and mix until just blended.
Using a small scoop (1 inch) drop the dough on a parchment lined cookie sheet, 3 inches apart (these cookies will spread a bit). (I’ve learned that having a scoop for these cookies is pretty important. You really want them to be consistent in size. If they’re too small, they will get too dark.)

Bake for about 10 minutes until the cookies are lightly golden around the edges. Cool on the pan for a couple of minutes before moving them to a cooling rack.
Fill the cooled cookies with the vanilla (or chocolate) filling and sandwich them together, pressing lightly to spread the filling to the edges of the cookie.

To make the fillings, mix the ingredients together until smooth and creamy. Add enough milk or cream to get a thick but spreadable consistency. You can fill them with jam if you prefer.