Preheat the oven to 180C/350F and line a muffin tin with liners. This recipe will make 12-15 muffins, so you may need two muffin trays.
Make the streusel topping first. Place butter, oats, flour, sugar and vanilla into a small bowl and rub together until lumps of dough form. You'll need the heat from your hands to create the lumps. I'd usually use a pastry cutter, but it doesn't work with streusel toppings. If one large lump of dough forms, crumble it in your hands. Otherwise just rub together until some smaller and large lumps form. Leave to one side.
Make the muffins: Place melted butter, sugar, vanilla and eggs into a large bowl and mix until smooth and slightly frothy.
Add in the flour, baking powder, bicarbonate of soda, salt, diced rhubarb and white chocolate chips. Mix until the batter starts to come together and then add the milk.
Continue to mix until you have a smooth mixture, with no lumps of flour. Don't beat the mixture or your muffins may end up rubbery.
Divide the batter between your liners. I used a 1.5tbsp cookie scoop and placed 3tbsp in each liner. You want to fill them level.
Using your fingers, crumble the streusel mixture on top of each muffin and if any bits don't adhere to the batter, gently press.
Place in the oven for 14-18 minutes, until risen, golden and an inserted skewer into the centre comes out clean.
Leave to cool in the pan for 10-15 minutes before transferring to a wire rack to cool completely.
To finish, you can melt some white chocolate and drizzle across each muffin. This is totally optional though!
Rhubarb and White Chocolate Streusel Muffins will keep in an airtight container, at room temperature for 3 days, or the fridge for 5. Best served at room temperature.