Mix the butter, sugar, confectioners sugar, and vanilla on medium speed until light and fluffy. Reduce to low speed and slowly add the flour and salt. When mixed, add the poppy seeds and continue mixing for 1 minute.
Remove the dough and divide it in two. Place one half between two sheets of parchment paper and roll until 1/4" thick. Using a cookie cutter to cut out the bottoms (removing unused dough, re-rolling and continue to make more bottoms until first half of dough is used up). Place all bottoms on parchment paper on a cookie sheet and freeze for an hour. Do the same thing with the other half of the dough and use a small shape cookie cutter to cut out centers.
To bake preheat the oven to 325 and bake the cookies about 10-12 minutes until light brown. Remove and cool on a wire rack and cool completely. With the top cookie, dust with confectioners sugar
Place about 1 teaspoon lemon curd on bottom cookie and then place top on and slightly press together.
Use a double boiler and bring water to simmer in pan making sure it won't touch the bottom of the top part.
Add in the top part all the ingredients minus the butter. Set it over the simmering water and whisk the mixture constantly until it reaches 170 degrees, remove from stove and add butter and stir until melted.
Strain the mixture through a fine-mesh sieve into a bowl.
Place this bowl inside another bowl that is filled with ice water.
Let the mixture stand at room temperature until it is cooled and thickened (this part can be done a day ahead and kept refrigerated until needed)