• Place a rack in the center of the oven.
• Preheat the oven to 180°C/350°F.

For the batter:

• Grease 15 medium-sized muffin cups with a small piece of butter, then lightly coat it with flour.
• Shake the muffin cups and invert them over a sink to get rid of the excess flour.
• In a bowl, sift together the flour, baking powder baking soda and salt.
• Place the butter and sugar in a bowl of an electric mixture, and beat on high speed for 30 seconds.
• Add the eggs, beating well after each addition.
• Slowly add the milk and vanilla and beat for 30 seconds. Scrape down the edges of the bowl.
• Stir in the flour mixture. Mix one more time with a wooden spoon to fully incorporate the batter.
• Coat the diced strawberries with the 1 tablespoon flour, and add to the batter. Stir one last time.
• Spoon the batter into the greased muffin cups, filling one third of each.

For the streusel:

• Mix together the walnuts, brown sugar, flour, and butter until crumbly. Sprinkle it evenly over the muffin cups.
• Bake for about 15 minutes, or until you insert a skewer in the center of a muffin and it comes out clean.

Meanwhile prepare the frosting:

• Place the butter in a bowl of an electric mixture. Add the icing sugar and diced strawberries and beat on high speed to mix well, obtaining a pink frosting.
• When the muffins have completely cooled down, spread the frosting equally on top of the muffins. You can decorate each muffin with a strawberry half, optional.