Spray inside of crock pot with non-stick cooking spray.

In a large microwave safe bowl, melt butter and chocolate chips in microwave on high for 1 minute. Stir until melted and if needed heat for an additional 15 seconds.

Stir in 3 tablespoons cocoa powder, 2/3 cup sugar, vanilla, salt, milk and egg yolk. Mix until completely combined.

Stir in flour and baking powder, mixing just until completely incorporated. Spread batter evenly in bottom of the prepared crock pot.

In a medium bowl, combine Remaining 6 tablespoons cocoa powder, remaining 1/3 cup sugar, brown sugar and hot water, mixing until combined. Gently pour over batter, do not stir in.

Cover crock pot with lid and turn crock pot to High setting. Cook, covered for 2 hours.

When cake is done (cake will be set and edges will be starting to pull away from sides of the pot), remove lid, being careful not to let condensation on lid drip onto cake.

Let cake sit for 30 minutes before serving. Top with a scoop of vanilla ice cream.