Preheat oven to 350 F. Line muffin tin with paper liners. Measure streusel ingredients into a medium bowl and mix with your fingers until combined and crumbly. Put streusel in fridge until further notice.
For the cupcakes, whisk together the flour, baking soda, baking powder, and salt. With an electric mixer, cream the butter and sugar until fluffy. Beat in the eggs, then the vanilla. Add the flour and sour cream and stir until just combined.
Divide batter among lined cups. Press about a tablespoon of streusel into each cupcake, then sprinkle another tablespoon of streusel on top of each. Bake, rotating pan if necessary, for 20 to 22 minutes, until cupcakes are set in the center and light golden brown.
Whisk together glaze ingredients and drizzle over warm cupcakes.
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