For the dough: In a large bowl or the bowl of your stand mixer, combine flour, 3/4 c sugar, yeast, 1 c warm milk, 1 egg, a dash of salt, and 5 TBSP melted butter.
Using the dough hook or your hands, mix the dough ingredients until they come together. Knead for another 5-10 mins.
Cover the bowl with a towel or plastic wrap and let the dough rise at room temperature or in a warm spot until it has doubled in size.
For the poppy seed filling: Grind the poppy seeds.
To a medium size pot, add a dash of salt, cornstarch, 2 c milk, and ¾ sugar. Using a wire whisk, mix until smooth. Whisk in the poppy seeds and the egg.
Over low-medium heat and while constantly stirring, bring this mixture to just boiling. Remove it from the heat and set it aside to let it cool down.
For the crumb topping: Combine 1 ¼ c flour, ½ c sugar, and ½ c butter in a medium bowl.
Using a fork, pastry blender, or your hands work this dough until you get a crumbly consistency. Set it aside.
Assemble the cake: Preheat the oven to 350˚FWhen the yeast dough has doubled in size, transfer it to a lightly floured work surface.
Using a rolling pin, roll the dough out to a big rectangle.
Place the poppy seed mixture directly on the dough, leaving about ½ inch on each side.
Starting with the long side, carefully roll up the dough into a long roll.
Transfer the dough roll onto a baking pan lined with parchment paper.
Starting from the middle, cut the roll in half lengthwise and braid it. Then cut it lengthwise in the other direction and braid it as well.
Gently press the streusel topping all over the roll.
Bake the cake in the preheated oven for 30-40 mins or until golden-brown on top. Remove it from the oven and let the cake cool down.
Finish the cake: Combine the lemon juice and powdered sugar to smooth glaze. Drizzle it generously over the poppy seed cake. Cut the cake into slices and enjoy!