This is a classic kolache filling, but also works well with Danishes and various kinds of sugar cookies.
1 cup poppy seeds
1 cup water
1/4 cup sugar
zest of 1/2 lemon
1 teaspoon cornstarch or flour
Bring the water to the boil and add the poppy seeds. Simmer covered for 20 minutes. Let the seeds stand about 15 minutes, drain them well (pour off any excess water and press them with the back of a spoon…or pour the while thing through a very fine med strainer or cheese cloth). Stir in the sugar, lemon zest and cornstarch (or flour).
If you can’t find poppy seeds in bulk anywhere, canned filling works fine. I stir in some fresh lemon zest to brighten the flavor.