Start by preheating your oven to 400°F (200°C). Prepare a muffin tin with liners or non-stick spray.
In a bowl, mix together the brown sugar, flour, and cinnamon for the streusel. Add in the cold butter cubes and use your fingers or a pastry cutter to create a crumbly texture. If desired, stir in chopped pecans. Set aside in the fridge.
In a large bowl, whisk the eggs, then mix in the melted butter, sugar, milk, and vanilla until well combined.
In another bowl, combine the flour, baking powder, and salt. Gradually add this to the wet mixture, stirring gently until just blended; avoid overmixing.
Spoon the muffin batter into the prepared muffin cups, filling them nearly to the top. Generously sprinkle the streusel topping over each one, gently pressing it into the batter.
Bake for 15 minutes until golden brown and a toothpick inserted in the center comes out clean. Let them cool in the pan for 5 minutes before transferring to a wire rack.