Grease slow cooker (4.5 to 6 L capacity) with cooking spray. In bowl, whisk flour with cocoa powder, baking powder and salt. Set aside.
In a separate bowl, beat butter and sugar using electric mixer on medium speed about 2 min. until light and fluffy. Beat in egg. Beat in sweet potato and vanilla.
With mixer on low speed, add the dry mixture to the sweet potato mixture in 3 parts, alternating with the milk in 2 parts, starting and ending with the dry mixture. Scrape bowl as needed between additions.
Scrape batter into slow cooker; smoothing top. Cover and cook on LOW setting 1 hr 10 min (or 10 min. longer) until firm and edges are browned. Let stand 15 min. in slow cooker. Invert ceramic insert of slow cooker to remove cake; cool completely on wire rack.
Meanwhile, make frosting by beating cream cheese with electric mixer until light and fluffy. Beat in butter completely. Then beat in icing sugar, vanilla and cardamom just until smooth.
Place cake on serving plate. Spread frosting over top and sides. Store refrigerated in airtight container up to 2 days.
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