Preheat oven to 325°F with convection // 350° without convection fan //  180°C // Gas Mark 4.

Line an 8x8 square cake pan with parchment paper. (*See notes)

In a medium bowl, combine flour, baking powder and salt. Whisk to combine. Set aside.

Beat butter and cream cheese together until uniform. Then add sugar and beat until light and fluffy (3 - 4 minutes).

Add egg and vanilla. Beat until combined.

Add dry ingredients and beat until just combined. Use a rubber spatula to scrape the bottom of the bowl to make sure everything is incorporated.

Press into prepared pan with lightly floured hands, or use a small offset spatula to spread the dough. Make sure to push dough into the corners and make it as even as possible in the pan.

Bake for 22 - 25 minutes, until a tester comes out clean and the edges are just barely golden brown.

Cool completely (at least 2 hours) before frosting.

While bars are cooling, make vanilla buttercream: Combine butter, salt and vanilla in a large bowl. Add 1 cup powdered sugar and beat to combine. Add 2 tbsp. milk and remaining powdered sugar. Adjust consistency by adding 1 tbsp. of milk, if needed. Cover with plastic wrap and set aside.

Spread buttercream over completely cooled bars and top with sprinkles. (Trim edges, if desired.) Cut into 9 large bars, or 16 bite-size pieces.