Beat butter and cream cheese together in a large microwave-safe bowl using an electric mixer until smooth. Add cocoa powder, walnuts, sugar substitute, heavy cream, vanilla extract, and raspberry extract. Stir fudge mixture slowly until well combined.
Microwave the fudge mixture on high, about 30 seconds. Continue blending with an electric mixer until smooth. Grease a 9x13 baking pan; spread mixture into the pan in 1 smooth layer. Cover with plastic wrap and refrigerate until set, at least 2 hours. Cut into bars.