Make the streusel topping by combining the flour, brown sugar, cinnamon, and softened butter in a small bowl. Either mash together or I like to use my fingers to combine into a mixture that has the consistency of 'wet sand' and can be clumped together. Pop in the fridge to chill until ready to use.
Preheat the oven to 350 degrees F and grease a large ramekin (10 ounces to the top rim and ~4.25 inches wide) with butter or cooking spray. I also like to add a circle of parchment paper to the bottom.
In a bowl, mix up the pumpkin puree, melted butter, egg yolk, and milk with a rubber spatula or whisk until combined. Mix in the sugar to combine.
Add the flour, baking powder, salt, cinnamon, and nutmeg and stir to just combine.
Pour the batter into the greased ramekin and crumble the streusel topping on top in big and small clumps.
Bake for 25-30 minutes until golden brown on top and a toothpick comes out with just moist crumbs, no wet batter. If your streusel topping is getting too dark, you can place a piece of foil on top.
Remove from the oven and cool slightly before eating. The coffee cake is easiest to remove from the ramekin when still slightly warm.
