Wash the artichoke and cut off all but 1 inch of the stem. Cut the artichoke exactly in half lengthwise (stem to tip). Season liberally with salt and pepper.
Place the trivet into the pressure cooker inner pot and place the artichoke halves cut side up into the pot. (Layer as appropriate if making more than one full artichoke. A 3 quart pressure cooker will hold 1 large cut artichoke. A 6 quart can hold up to 3 cut artichokes. )
Pour water and vinegar into the pressure cooker inner pot. Pressure cook on high pressure for 17 minutes then release the steam. The base of the artichoke should be fork tender, but not mushy. If it's too firm, add another minute of cooking time.
Make the basting sauce while the artichoke is in the pressure cooker. Melt the butter and stir in the garlic powder until blended. Set aside.
Make the dipping sauce while the artichoke is in the pressure cooker. Mix together the mayonnaise, sriracha, dijon mustard and pickle juice until well blended. Set aside.
Carefully remove the artichokes from the pressure cooker when the cooking cycle is complete and the steam is released. Place on a cutting board, cut side up. Scoop out the inner-most part of the artichoke. You should remove all of the spiny, "hairy" parts using tongs or a large spoon. It should scrape out fairly easily. (See YouTube video for a demo.) Baste the cut side of the artichoke with the melted butter/garlic mixture using a silicone brush.
Turn the grill on to high (approximately 400 degrees). When it is hot, add the artichokes, cut side down. Be careful because it will flame due to the butter. This is ok! This is what gives it the char flavor and texture you want. Flip to the other side when it is charred to your liking (about a minute or two). Char the other side for another minute. Remove and enjoy with the dipping sauce.