Preheat the oven to 220°C/200°C fan/425°F/gas mark 7. Line a 12 hole muffin tin with 9-10 muffin cases (or ideally space them across two muffins trays if you have an extra one).
To make the streusel topping, mix together the flour, sugar and cinnamon in a bowl then rub in the butter using your fingertips until no lumps of butter remain and the mixture is clumping together. Place the bowl in the fridge while you make the muffin batter.
To make the muffin batter, whisk together the milk, yogurt, oil, sugar, vinegar and vanilla extract in a large bowl until well combined.
Whisk in the cinnamon, baking powder, bicarbonate of soda and salt followed by the flour. Be careful not to over-beat the batter, just mix it until no streaks of flour remain. Gently fold in the diced pears.
Divide the batter between the muffin cases, filling them almost to the top. An ice cream scoop is handy for this. Scatter over the streusel mixture.
Bake for five minutes then turn the oven down to 180°C/160°C fan/350°F/gas mark 4. Bake for a further 18-20 minutes, until a skewer inserted into the centre comes out clean.
Allow the muffins to cool in the tin for 5 minutes, then transfer them to a wire rack and leave to cool completely. Once cold store in an airtight container. Best eaten on the day they are baked.