Make streusel- in a bowl, melt the butter and let it cool for a few minutes. Mix in the sugar, flour, and cinnamon until combined. Place this in the fridge while making the rest of the cake.
8 tbsp unsalted butter, ¾ cup granulated sugar, 1 cup all-purpose flour spooned and leveled, 1 ½ tsp cinnamon
Preheat oven to 350 F and grease an 8x8 square pan. Line with parchment paper for easy removal. Set aside.
In a small bowl, mix together the flour, baking powder, and salt.
2 cups all-purpose flour, 2 tsp baking powder, ½ tsp salt
In a separate large bowl, use an electric mixer to cream sugar and butter together until smooth, about 1-2 minutes. Add the egg, egg yolk, vanilla extract, and sour cream, mixing until combined.
¼ cup unsalted butter, ¾ cup granulated sugar, 1 large egg, ¼ cup sour cream, 2 tsp vanilla extract, 1 large egg yolk
Add the flour mixture and milk in alternating additions, starting and ending with the flour mixture. Mix until just combined, be careful not to overmix here.
½ cup whole milk
Pour half the batter in the greased pan. Sprinkle a little less than half of the streusel on top of the batter. Carefully spread the rest of the batter on top. Top evenly with the remaining streusel, and then drizzle the caramel sauce on top of that (I heated my caramel in the microwave for a few seconds to get it to be more runny)
3 tbsp caramel sauce
Bake for 35-40 minutes, or until a toothpick comes out clean. Allow the cake to cool in the pan for 10-15 minutes, and then allow to cool completely.
Once cooled, top with icing- in a small bowl whisk together the powdered sugar and milk until smooth. Drizzle the sugar glaze over the cake, slice, and enjoy.
½ cup confec sugar, ¼ tsp vanilla, 1 ½-2 teaspoons milk or cream