Preheat oven to 220°C (428F) and line a muffin pan with muffin liners.
Mix chai blend ingredients in a medium-sized bowl and set aside.
In a medium-sized bowl, sift and mix flour, baking powder, and salt and set aside.
Place sugars and 2½ tsp. chai blend in a large bowl of a stand mixer fitted with with paddle attachment. Add vegetable oil and mix on medium speed until well-combined.
Add Greek yogurt and mix for another minute.
Add dry ingredients in two additions, alternating with milk and mix on medium speed until just combined. Do not overmix.
Divide batter between 8-10 muffin liners, filling all the way to the top for a large bakery-style muffin or ¾ full for a smaller muffin. Top with 2-3 tsp. streusel topping.
To make streusel topping: In microwave, melt butter in a medium-sized microwave-safe bowl. Stir in ½ to ¾ tsp. of the remaining chai blend, 1 tbsp. granulated sugar and 1 tbsp. brown sugar. Add 2-4 tbsp. flour and stir until a crumbly streusel forms.
Bake muffins for 7-8 minutes, then lower the oven temperature to 175°C (350F) without opening the oven and bake for a further 8-12 minutes until muffin tops are golden brown and a toothpick inserted into the center comes out clean.
Allow to cool in muffin pan for 5-10 minutes, then transfer to cooling rack to finish off cooling.
To make the glaze: Mix powdered sugar and chai blend. Stir in heavy cream/milk until desired consistency is reached. Drizzle over cooled muffins. Allow to set and serve.