Preheat your oven to 350°F (175°C) and prepare an 8x8-inch baking dish by greasing it or lining it with parchment paper.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt.

Add the softened unsalted butter, egg, and milk to the dry mixture and blend until it forms a crumbly dough.

Press two-thirds of the dough into the bottom of the prepared baking dish and set aside the remaining dough for topping.

In another bowl, beat the cream cheese until smooth, then mix in the powdered sugar and vanilla extract until well combined.

Spread the cream cheese mixture over the crust, then sprinkle the chopped fresh rhubarb evenly on top.

Crumble the reserved dough over the rhubarb layer evenly.

Bake in the preheated oven for 35 to 40 minutes, or until the topping is golden brown and the filling is set.

Allow to cool completely in the pan on a wire rack, then refrigerate for at least 2 hours before slicing.

Slice the bars into squares and serve chilled, optionally with vanilla ice cream.