In a large mixing bowl, combine the flour, granulated sugar, cocoa powder, baking powder, and salt. Whisk until well blended.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients. Mix until just combined, being careful not to overmix.
Fold in the semi-sweet chocolate chips.
Pour the batter into a greased 6-quart crockpot, spreading it evenly.
Carefully pour the hot water over the batter in the crockpot. Do not stir; the water will create the lava effect as it cooks.
Cover the crockpot with the lid and cook on high for 2 to 2.5 hours, or until the edges are set but the center is still slightly gooey.
Once done, turn off the crockpot and let it sit for 10 minutes before serving.
Serve warm, dusted with powdered sugar if desired.