Take your muffin pan and line it with liners. Grease the liners generously with butter and set them aside.
In a large bowl beat butter and both the sugars together using an electric hand mixer on medium speed until the mixture turns creamy and fluffy. This might take around 2-3 minutes.
In another bowl, mix together the dry ingredients – Flour, salt, and baking powder. (This is your dry mixture)
Next, add the eggs and vanilla to the butter-sugar mixture and beat on medium speed until everything is combined. (This is your wet mixture, it might look grainy but that’s fine)
Gently fold the dry mixture into the wet mixture in three equal and separate additions, alternating with the milk and ending with the dry mix. Make sure that you only mix till the flour disappears. Do not overmix.
Toss the diced strawberries with ½ tbsp of flour; doing so will keep them from sinking at the bottom while baking. Reserve ½ cup of strawberries to sprinkle on top and add the remaining to the batter. Fold the strawberries gently using a silicone spatula until evenly distributed. Do not overmix.
Preheat the oven to 425 degrees F / 220 degrees C and let the batter rest while you preheat the oven.
Divide the batter between each muffin cup, filling almost to the top. Tap the muffin pan gently on a hard surface. Scatter the reserved strawberries evenly on top, then sprinkle the brown sugar on the top for a nice crunch.
Bake in a preheated oven at 425 degrees F for 6 minutes, then keep the muffins in the oven and reduce the temperature to 375 degrees F / 190 degrees C and bake for another 18-20 minutes or until the muffins turn golden brown on top and the toothpick inserted in the middle comes out clean.
Allow the muffins to cool in the pan for 5 to 10 minutes before serving.