Preheat your oven to 325°F (163°C) and prepare an 8-inch square baking pan by greasing it or lining it with parchment paper.

In a medium bowl, whisk together flour, baking soda, and salt until combined.

Cream unsalted butter, granulated sugar, and brown sugar until light. Add egg, vanilla extract, and sour cream, mixing until creamy.

Gradually add the flour mixture to the wet ingredients on low speed until just combined.

Divide the dough in half and fold in cocoa powder into one half to create the chocolate layer.

Press the chocolate dough into the bottom of the prepared pan, then spoon and spread the vanilla layer on top.

Bake for 35 to 40 minutes, until the edges are barely golden and a toothpick comes out clean.

Cool the bars on a wire rack.

Mash raspberries and strain through a sieve to collect juice, mixing with confectioners' sugar for a thick glaze.

Pour and spread the raspberry glaze over the cooled bars, optionally adding sprinkles before serving.