Begin by preheating your oven to 350 degrees F. Grease and flour a bundt pan. (Alternatively you can use a 10-inch tube pan or a 13x9-inch pan.) Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and sugar until light (about 4-5 minutes). Add the eggs one at a time, then add the vanilla and sour cream. In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. With the mixer on low, add the flour mixture to the batter until just combined. Then, with a spatula, gently fold in the sprinkles.
Next, prepare the streusel. Place the brown sugar, flour, cinnamon, salt, and butter in a bowl. With your fingers, pinch the mixture together until it forms a crumble.
Into your prepared pan, spoon in half of the batter and level with a spatula. Then, sprinkle with 3/4 of the streusel mixture. (Only use 1/2 if you plan to have your crumb layer be on the top of your finished cake.) Spoon the rest of the batter into the pan, evenly spread it out, and top with the remaining streusel. Bake for 50-60 minutes or until a toothpick inserted near the center comes out clean.
Allow the cake to cool on a wire rack for about 30 minutes. Then, remove the cake from the pan and place on a serving plate. Allow the cake to cool.
In the meantime, prepare the funfetti glaze. In a bowl, whisk together the confectioner's sugar, melted butter, vanilla, salt and milk. You'll want the glaze to be as thick as possible while still being a bit runny. If it's too runny, add more confectioners sugar; if it's too thick, add more milk.
Once the cake has cooled (It can still be slightly warm.), drizzle as much of the glaze over the cake as you'd like. Then, add more sprinkles before the glaze has set.