Start by making the raspberry sauce. In a small pot, combine raspberries and water over low heat. Simmer until the raspberries are soft enough to mash easily. Stir in your sweetener, then whisk in glucomannan (or xanthan gum) until the sauce thickens. Set aside to cool completely.
Preheat your oven to 325°F (160°C). Lightly grease a 9x9 inch square baking pan. In a bowl, whisk together almond flour, cocoa powder, sweetener, and salt. Pour in the melted butter and mix until crumbs form. Press this mixture firmly and evenly into the bottom of your prepared pan. Bake for 12 minutes, then let it cool while you prepare the cheesecake filling.
Reduce the oven temperature to 300°F (150°C). Gently melt the white chocolate and cocoa butter (or coconut oil) in a heatproof bowl set over a pot of barely simmering water, stirring constantly until smooth. Be careful not to let it seize! Once melted, remove from heat.
In a large mixing bowl, beat the softened cream cheese and sour cream until perfectly smooth. Mix in the powdered sweetener until combined, then gently fold in the melted white chocolate mixture. Finally, add the egg and vanilla extract, beating just until everything is combined – don't overmix!
Spoon about half of the cheesecake batter over the cooled crust, then drizzle half of your cooled raspberry sauce on top. Repeat with the remaining batter and sauce. Use a knife or skewer to gently swirl the raspberry sauce into the cheesecake for a beautiful marbled effect. Carefully shake the pan slightly to level the filling.
Bake for 30-35 minutes, or until the edges are set but the center still has a slight jiggle. Let the bars cool completely at room temperature, then chill in the refrigerator for at least 3 hours before slicing and serving.