Grind the poppy seeds in a mill or coffee grinder.
Combine the milk, butter, salt, and sugar in a saucepan. Turn the heat to low and stir the mixture often until the sugar dissolves.
Slowly, whisk in the eggs and continue to stir the mixture until it begins to thicken (it should resemble the consistency of sweetened condensed milk). Note: If you let the mixture sit too long, the eggs will cook and begin to scramble.
Add the poppy seeds into the sauce pan and stir to combine. Remove the poppy seed filling from heat and allow it to cool at room temperature.
Once it's completely cooled, transfer the filling to an airtight container and store it for up to a week. Enjoy it spread on toast or added to your favorite baked goods!