Preheat your oven to 375°F. Line a standard muffin tin with paper liners and lightly spray them with nonstick spray to help the muffins release cleanly.
In a large bowl, use a hand mixer or stand mixer to beat the melted butter and brown sugar until smooth.
Add in the eggs, vanilla extract, pumpkin puree, and Greek yogurt. Mix until fully combined and creamy.
In a separate bowl, whisk together the flour, baking soda, salt, cinnamon, and pumpkin spice.
Gently fold the dry ingredients into the wet mixture using a spatula or wooden spoon. Mix just until combined - don’t overmix. Let the batter rest while you make the streusel.
In a medium bowl, stir together the flour, brown sugar, and cinnamon. Drizzle in the melted butter and mix with a fork until it forms coarse crumbs.
Fill each muffin cup about ¾ full with the batter. Sprinkle the streusel mixture evenly over each muffin top.
Bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean. Let the muffins cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
Beat the softened cream cheese in a bowl until smooth. Add in the confectioners’ sugar, vanilla, and 1 tablespoon of milk. Beat until creamy, adding more milk as needed for a spreadable consistency.
Once the muffins are fully cool, pipe or spread the cream cheese frosting on top. Enjoy immediately or refrigerate for a chilled treat.