• Preheat oven to 425°F. Line a standard 12-cup muffin tin with paper liners.

Raspberry Swirl

• In a small saucepan over medium heat, combine raspberries, sugar, lemon juice, and cornstarch. Cook, stirring gently, until the raspberries break down and the mixture thickens to a jam-like consistency, about 5 to 7 minutes. Lightly mash some of the berries with a spoon, but leave small pieces for texture.
• Remove from heat and allow to cool slightly while preparing the batter.

Muffin Batter

• In a medium bowl, whisk together flour, sugar, baking powder, and salt.
• In a separate bowl, whisk together melted butter, eggs, vanilla extract, and milk.
• Add the dry ingredients to the wet ingredients and stir just until combined. Do not overmix.
• Gently fold in the fresh raspberries. If they are large, lightly break a few in half before folding for better distribution.

Crumb Topping

• In a small bowl, combine flour, granulated sugar, brown sugar, lemon zest, and salt. Stir in melted butter until crumbly. The mixture should form large clumps when pressed together.

Assembly

• Fill each muffin cup nearly to the top with batter.
• Spoon about 1 tablespoon of raspberry swirl over each muffin. Use a knife to gently drag through once or twice for a marbled effect.
• Top generously with crumb topping, lightly pressing it into the batter so it adheres.
• Bake for 10 minutes. After 10 minutes, without opening the oven door, reduce the oven temperature to 375°F and bake for another 19 to 21 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs but no wet batter.
• Let the muffins cool in the pan for 5 to 10 minutes. Then, remove the muffins from the muffin tin and place them onto a wire rack to cool completely.