In a mixing bowl, combine all-purpose flour, light brown sugar, and salt until well blended. Cut in cold unsalted butter until the mixture resembles coarse crumbs. Stir in the chopped hazelnuts and refrigerate for about 15 minutes.

Preheat your oven to 350°F (175°C) and prepare a 9"x13" pan by greasing it lightly with butter or non-stick spray.

In a mixing bowl, combine the cream cheese, granulated sugar, all-purpose flour, egg, and vanilla extract. Beat until smooth and creamy.

Whisk together the all-purpose flour, cocoa powder, baking powder, baking soda, and salt in another bowl until well combined.

In a large bowl, beat the unsalted butter with the brown sugar and granulated sugar until light and fluffy, about 3-5 minutes. Add the egg and other wet ingredients, blending until fully incorporated.

Gradually mix in the dry flour mixture and the yogurt until just combined. Transfer the thick batter to the prepared pan, smoothing it evenly.

Spoon dollops of the cream cheese mixture over the chocolate batter and gently swirl with a butter knife.

Sprinkle the chilled crumb topping over the cream cheese layer and bake for 40-45 minutes, or until a toothpick comes out clean.

Allow to cool in the pan for about 20-30 minutes before slicing into squares.