Preheat oven to 350°F. Line a 9-inch square pan with parchment paper or lightly grease

In a large bowl, whisk together melted butter, brown sugar, and granulated sugar until smooth. Add eggs + egg yolk and vanilla, whisk until fully combined.

Stir in flour, cornstarch, cinnamon, baking powder, and salt. Mix just until a thick dough forms.

Press half of the dough evenly into the bottom of your prepared pan.

In a small bowl, mix brown sugar, cinnamon, and melted butter. Sprinkle and gently spread the filling evenly over the base layer.

Take the remaining dough and drop small pieces over the filling. Use your fingers to gently press and spread it into a rough top layer. Important: Do not fully spread like frosting or you’ll mix into the filling. Think “patchwork” that you lightly press together.

Bake for 35–40 minutes. You’re looking for: Set edges. Center that looks slightly soft but not jiggly. Do not overbake. They will finish setting as they cool.

In a small bowl, whisk together powdered sugar, cinnamon, vanilla, and cream. Consistency should be smooth but pourable, but not watery.