In the bowl of a stand mixer whisk together the flour, sugar, yeast, lemon zest, and poppyseeds.
Sprinkle in the salt. Then add in the warm milk, two eggs, and vanilla extract.
Use a dough hook to knead everything together on medium low speed for 15 minutes until the dough has smooth edges and the bowl is clean.
Add half of the butter in and knead again until incorporated (about 5 minutes). Add in the second half of the butter and knead once more until smooth. The dough should pass the window pane test *see notes section for more info.
Shape the dough into a ball and place in a lightly greased bowl that is at least twice the size of the dough.
Cover the bowl with plastic wrap or a towel and place somewhere warm to rise for 1 hour. The dough should double in size.
Grind the 50 grams of poppyseeds for the filling, I recommend a spice grinder or coffee grinder for this. Pour into a medium heat safe bowl.
In a medium pan, add in the butter. Place over medium low heat (medium if using electric and your stove isn't very hot).
Cook until bubbling, then use a rubber spatula to stir it gently until brown bits form. Keep scraping and stirring the pan until the butter is heavily speckled with brown bits but not burnt.
Turn the heat off and immediately pour into the bowl with the ground poppyseeds. Stir to combine.
Then add in the sugar, lemon juice, and salt and stir again.
Place the bowl in the fridge or in an ice bath to cool it down faster. Be sure to stir it every five minutes to keep the ingredients incorporated. Cool until spreadable and then leave at room temperature.