Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners or grease it lightly.
In a medium bowl, combine the 1/2 cup brown sugar, 1/2 cup all-purpose flour, and 1 tsp ground cinnamon.
1/2 cup brown sugar, 1/2 cup all-purpose flour, 1 tsp ground cinnamon
Cut in the cold, cubed 1/4 cup unsalted butter using a pastry cutter or two forks until the mixture resembles coarse crumbs. Set aside.
1/4 cup unsalted butter
In a large bowl, whisk together 1 1/2 cups all-purpose flour, 1 tsp salt, 1 1/4 cups sugar, 3/4 tsp baking powder, and 1/2 tsp baking soda.
1 1/2 cups all-purpose flour, 1 tsp salt, 1 1/4 cups sugar, 3/4 tsp baking powder, 1/2 tsp baking soda
In a separate bowl, cream together the 1/2 cup peanut butter and softened 1/2 cup unsalted butter until smooth and fluffy.
1/2 cup peanut butter, 1/2 cup unsalted butter
Add the 1/2 cup sour cream, 2 tbsp whole milk, 2 eggs, and 1/2 tbsp vanilla extract to the peanut butter mixture. Beat until well combined.
1/2 cup sour cream, 2 tbsp whole milk, 2 eggs, 1/2 tbsp vanilla extract
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
Spoon half of the batter into the prepared muffin tin, filling each cup about 1/3 full.
Sprinkle half of the streusel topping evenly over the muffin batter.
Spoon the remaining batter over the streusel.
Sprinkle the rest of the streusel topping evenly over the muffin batter.
Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for about 5 minutes, then transfer them to a wire rack to cool completely.