Preheat your oven to 350°F and line a large baking sheet with parchment paper.
In a large bowl, cream together the softened butter, ½ cup brown sugar, and granulated sugar until fluffy.
Mix in the egg, sour cream, and vanilla.
In a separate bowl, whisk together the flour, baking powder, salt, and 1 tsp cinnamon. Slowly add to the wet ingredients until a thick dough forms.
In a small bowl, make the streusel: combine cold cubed butter, flour, the remaining ¼ cup brown sugar, and 1 tsp cinnamon. Use a fork or pastry cutter until crumbly.
Scoop large balls of dough (about 3 tbsp each) and place on the baking sheet. Use your thumb to make a deep well in each ball.
Fill the wells generously with the streusel mixture.
Bake for 11–14 minutes until the edges are golden. Let cool on the pan for 10 minutes.
Whisk together the glaze ingredients and drizzle over the cooled cookies.