In a bowl, rub together the granulated sugar and vanilla sugar using your fingers.
Add oil, eggs (and optionally one egg white), vanilla syrup, and whisk for about 1–2 minutes until light and slightly fluffy.
Add sour cream (or yogurt) and lemon juice. Mix well.
Then add flour, potato starch (optional), and baking powder. Mix until just combined.
Finally, fold in the chopped strawberries.
Place tall muffin liners into every second cavity of a muffin tray (or use baking paper to create taller liners).
In a separate bowl, prepare the crumble by combining flour, sugar, and cold butter. Rub with your fingers until a crumb forms.
Sprinkle the crumble over the muffins.
Bake at 210°C for 5 minutes, then reduce the temperature to 180°C and bake for another 15–20 minutes (about 17 minutes total after reducing the temperature).