Preheat your oven to 350 degrees F. Line a muffin tin with cupcake liners or generously spray the cups with cooking spray.

In a small bowl, mix together the brown sugar, 1/2 cup flour, and melted butter until crumbly. Set this mixture aside for later.

Using a stand mixer with a paddle attachment or a large mixing bowl with a hand mixer, beat the softened butter and granulated sugar until combined.

Add the eggs and sour cream to the mixture and beat on medium-low speed until creamy.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the butter mixture, beating on medium-low speed until just combined.

Carefully fold in the mulberries.

Evenly distribute the batter into the 12 muffin cups. Sprinkle the crumble mixture on top of each muffin.

Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted into the center of a muffin comes out clean.