In a food processor, process the sugar and butter until incorporated.
Mix the flour, salt, and baking powder in a bowl then add it to the food processor followed by the egg and vanilla extract.
Pulse the mixture until a dough is formed.
Wrap the dough with plastic wrap and refrigerate for a minimum of 1 hour. Remove from the refrigerator 10-15 minutes before baking.
Preheat oven to 375F Line an 8x8-inch baking pan with parchment paper and grease the sides.
Roll the dough and flatten it evenly inside the baking pan. Using a fork prick it thoroughly and bake it for 15 minutes. Remove from the oven and set aside to cool.
Make The Toppings
Place the poppy seeds filling ingredients in a food processor and process until a smooth paste is formed.
Use a fork to mix all of the crumbs topping together until you have a lumpy mixture.
Assemble and Bake
Preheat your oven to 350F (180C)
Evenly spread the filling and sprinkle the crumbs on-top.
Bake for 30-35 minutes until the streusel is slightly golden brown. Allow the bars to completely cool before slicing.
Notes
Use a store-bough filling if available to you. (1-2 cups depending how thick you wish your filling to be)
For a thicker layer of filling, double the filling layer.
Make sure the ingredients are at room temperature, and always use a kitchen scale to measure the ingredients.
Brush the topping with beaten egg yolks for a more vibrant topping color.
The cake is ready when the topping is golden brown, and the edges of the crust should be set. If unsure, insert a toothpick into the filling; it should come out mostly clean.
You can use ground poppy seeds instead of whole poppy seeds but note it will enhances the bitterness of the seeds.
Add 1/2 cup of fine ground almonds to the flour mixture for an extra nutty flavor.
This recipe can be used to make one 8x8-inch cake, or 9x9-inch cake, and can be sliced into 12-16 pieces. You can also use an8, or 9 inch springform pan. Another option is to se a 9x13-inch baking pan, it will be thinner and I recommend doubling the filling recipe. Make sure the pans are at least 2.5 heigh.