Preheat the oven to 374°F (190°C) or 338°F (170°C) for a fan-assisted oven. Line a muffin tin with paper liners.
In a large bowl, whisk together the flour, brown sugar, baking powder, and salt.
In a separate bowl, whisk together the kefir, sunflower oil, egg, vanilla extract, and lemon zest until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Do not overmix.
Fold in the raspberries carefully to avoid crushing them.
Divide the batter evenly among the muffin cups, filling each about two-thirds full.
Sprinkle sugar over the tops of the muffins.
Bake for 18–20 minutes or until a toothpick inserted into the center comes out clean.
Cool the muffins in the tin for 5 minutes, then transfer to a wire rack to cool completely.
Notes
Use Room Temp Ingredients: I always let the kefir and egg sit out for a bit; everything blends better, and the muffins bake up fluffier.Don’t Overmix: I stop stirring as soon as the flour disappears. It keeps the muffins soft and tender every time.Fold Gently: I take my time folding in the raspberries so they stay whole and don’t streak the batter too much.Sugar on Top: I never skip the sugar sprinkle; it gives the muffins that perfect little crunch on top.Best Warm: I like to eat one while it’s still warm from the oven. The berries are juicy, and the top has the perfect texture.