Prepare Filling (use strawberry jam if you want to shorten prep time)
Combine the diced strawberries, granulated sugar, cornstarch, and lemon juice in a small saucepan.
Cook over medium heat, stirring frequently, until the mixture thickens and the strawberries soften, about 5-7 minutes. Use a pastry cutter or fork to break apart the strawberries. Remove from heat and let cool completely.
CRUMBLE
Combine sugar, brown sugar, cinnamon, salt, flour, and melted butter, stirring with a fork until crumbly and set aside.
DOUGH
Slice room-temperature butter into small chunks and place in the bowl of a stand mixer.
Add sugar to the bowl and cream with the butter until light and fluffy.
Add the egg and vanilla to the bowl. Beat again till creamy and well mixed.
Add flour and salt to the bowl. Mix on low until just combined
Remove the dough from the bowl and knead with your hands until a smooth dough ball forms, but don't overwork the dough.
Form the dough into a flat disk, wrap it in plastic wrap, and place it in the refrigerator for at least 3 hours.
Lightly flour a smooth, clean surface. Unwrap the dough disk and place it on the floured surface. The dough will be very firm after chilling.
Remove dough from the fridge use a rolling pin to roll the dough out until it's about 1/4 inch thick. If it starts to crack, use your finger tips with a bit of water to smooth out the cracks.
If the cookie dough gets too soft, put back into the fridge for 15-20 minutes. If it's too soft, it won't stay folded into triangles while baking.
Use a 3-inch round cookie cutter for smaller cookies or a 3.5-4 inch cutter for larger cookies or rim of a glass to cut circles out of the dough, and spoon two tsp of strawberries to the center of each circle.
To shape the hamentashen into a triangle, fold the top edges of the circle and pinch together to form the top of a triangle, leaving the filling exposed, then fold up the bottom and pinch the right and left seams on the bottom tightly together so it's in the shape of a triangle.
Sprinkle 1 tbsp of crumble on top of the cookies.
Place on lightly greased baking sheets or lined with parchment paper and bake at 350 degrees for 10-15 minutes or until golden brown.