Coat 4-quart or larger slow cooker with cooking spray. Mix flour, sugar, cocoa powder, baking powder, and salt in bowl.
Combine milk, margarine, egg, and vanilla extract in another bowl. Mix in flour mixture until smooth, then stir in chocolate chips. Spread batter evenly in slow cooker.
Cover. Cook on low 2 to 3 hours, or until wooden pick inserted in center comes out clean. Let cool, uncovered, 15 minutes. Remove insert from cooker and invert contents over rack, or let cool completely in cooker before cutting into 12 squares.
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Slow Cooker Brownie Cake Squares (cont.)