Lightly grease a 9-inch (23 cm) round springform pan with butter or nonstick cooking spray and lightly flour. I recommend wrapping the outside of the pan in tin foil, as some springform pans can leak. Set aside.
Stir together the flour, brown sugar, white sugar, and cinnamon in a large bowl.
Use a pastry blender (or 2 forks) to cut the butter into the flour mixture. When you're done, the mixture should look crumbly and there will be very small pieces of butter no bigger than a pea. Place in the fridge as you make the cake batter
Coffee Cake Batter
Whisk together the flour, baking powder and salt in a medium-sized bowl.
Beat together the butter and sugar in a large bowl using an electric mixer. Beat the mixture until fluffy.
Beat in the eggs 1 at a time. Then mix in the vanilla extract and sour cream.
Sift in about ½ of the flour mixture and mix on low speed until combined. Mix in the milk, followed by the rest of the flour mixture. Stop mixing as soon as everything is combined.
Pour/spoon the batter into the prepared pan.
Place the raspberries on top of the cake batter to create an even layer. I typically flatten them slightly. Then sprinkle the streusel on top of the raspberries by crumbling it in between your fingers & fists until an even layer of crumble covers the raspberries.
Bake in your prepared oven for 50 to 60 minutes, or until an inserted toothpick in the center of the cake comes out clean.
Remove from the oven. Gently trace around the edges of the pan with a thin knife, then allow the cake to cool fully in the pan.
Glaze and Serving
Whisk together 2 tablespoons of milk (or cream) with the powdered sugar. Whisk in a little more milk or powdered sugar as needed - it should be a consistency that you can drizzle.
Ensure the cake is fully cooled (the pan should feel room temperature), then unclamp the outer ring of the springform pan.
Drizzle the cake with the glaze.
Slice with a sharp knife, ensuring that you slice all the way through the cake.
For the vanilla cake layer, the butter, eggs, sour cream and milk should all be room temperature before getting started.
Sour cream can be substituted with plain Greek yogurt if needed.
If you don't have buttermilk, 1% or 2% milk can be used if needed.
I don't recommend substituting both sour cream and buttermilk.
Store cake covered at room temperature for up to 3 days, or in the fridge for up to 4 days. Bring to room temperature before serving. Cake can be wrapped tightly and frozen for up to 2 months. Then thaw in the fridge overnight and bring to room temperature (by setting on the counter) before serving.
Nutrition information is an estimate only and based on 1 slice, assuming the cake is sliced into 10 equal pieces.