Bitter chocolate pudding
Just a little bit of expectation management: stevia has a naturally bitter aftertaste, and depending on the amount and type of extract, it can be very bitter. You might also be over using stevia if you're just substituting it without balancing your recipe. 

When using sweeteners to replace sugar:


Make sure to buy a sweetener that is already balanced to replace sugar on a 1:1 ratio so you don't overuse it and also don't lose the bulk of solid that sugar provides
If the recipe allows you to add sugar to taste (since this is the case) because it doesn't rely on the sugar for bulk, texture or for balancing liquids, add it to taste and if possible, make it a tad undersweet. Less is more when we're talking about sugar and salt