Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish or line it with parchment paper.

In a small bowl, mix together the granulated sugar and 1 1/2 teaspoons of ground cinnamon; set aside.

In a medium mixing bowl, whisk together the flour, baking powder, salt, and cream of tartar.

In a large mixing bowl, cream the unsalted butter with the granulated sugar until light and fluffy, about 2-3 minutes.

Add eggs, milk, vegetable oil, and vanilla extract, and mix on low speed until just combined.

Gradually add the dry mixture to the wet ingredients and mix on medium speed for about 30 seconds, avoiding overmixing.

Pour half of the batter into the prepared baking pan and spread it evenly.

Sprinkle half of the cinnamon-sugar mixture over the batter, then add the remaining batter, spreading it gently.

Sprinkle the remaining cinnamon-sugar over the top.

In a small bowl, mix brown sugar, 1/4 cup flour, and 1 teaspoon cinnamon. Add the cubed cold butter and mix until crumbly.

Evenly sprinkle the crumb mixture over the top.

Bake in the preheated oven for 35-40 minutes, covering with foil if it browns too quickly.

Cool in the pan for 10-15 minutes before glazing.

For the glaze, mix confectioners' sugar and milk until smooth. Drizzle over the warm cake.