In a large bowl, measure out the butter and peanut butter. Microwave in 30-second intervals until completely melted and smooth; should take about 1 1/2 minutes.: The warm, nutty aroma of peanut butter mingling with the creamy scent of salted butter signals the start of something comforting. As the butter melts and the peanut butter loosens, you will see a glossy pool forming, with tiny bubbles at the surface if it becomes slightly too hot. I like to stop microwaving when there are still a few soft lumps, then stir until perfectly smooth, because residual heat finishes the job and prevents overheating. Overheating will make the oils separate, so avoid long, continuous microwaves, and use 30-second bursts instead. Stirring smoothly also helps the mixture become homogeneous, which ensures even sweetness and texture later.
Stir in powdered sugar and rice Krispies and gently incorporate until fully mixed together.: At this stage you will notice the powdered sugar quickly dulls the sheen of the melted fats, transforming the mix into a doughy, pliable mass. The rice krispies cereal adds visual flecks of white and audible crackle when pressed between fingers. I fold slowly to keep the cereal pieces intact, since crushing them would flatten the texture. If the mixture feels too loose, a brief chill will firm it up; if it is too dry, a touch more melted butter smooths it out. Avoid vigorous mixing, which can overwork the sugars and make the balls dense.
Using a 2 teaspoon cookie scoop, scoop mixture into 24 even balls. Roll in between palms to smooth completely. Place onto a parchment (or wax) paper-lined baking sheet. Freeze 10 minutes.: When you scoop, the surface should look matte and slightly firm, not wet. Rolling the mixture between your palms warms the dough just enough to round it, creating a seamless exterior. As you work, the scent becomes sweeter and the texture more compact. Spacing them evenly on parchment prevents sticking and makes dipping easier. The freeze for about 10 minutes firms the centers, which helps the chocolate adhere cleanly. A common mistake is skipping the chill, which leads to misshapen balls and excessive chocolate absorption.
While peanut butter balls are freezing, melt chocolate and vegetable oil together in a small microwave-safe bowl. Microwave in 30-second intervals until completely melted and smooth.: The melted melting chocolate or chocolate chips should be glossy and fluid, with a silky sound when you stir. Adding the vegetable oil tames thickness and improves sheen. Heat gradually and stir in between intervals, because overheated chocolate can become grainy and dull. If you notice small lumps, continue brief heating and stirring until smooth. Avoid adding cold liquid directly to hot chocolate, as that can cause seizing. The goal is a coating that flows like warm syrup.
Drop the frozen balls one at a time into the melted chocolate. Use a fork to roll ball evenly in chocolate. Lift ball out of the chocolate with the prongs of the fork and tap gently to remove any excess chocolate. Place back on parchment paper.: The first dipped ball often tells you everything. As the cold center meets the warm chocolate, you will hear a faint, satisfying snap as the coating firms quickly at the surface. The fork lets excess chocolate drip back into the bowl, and gentle taps keep the coating thin and even. If the chocolate pools at the bottom when you place the ball back down, a slight tilt or a second tap will tidy the edges. Beware of leaving the balls in the chocolate too long, which can trap air bubbles or create an overly thick shell that hides the center texture.
Continue coating balls in chocolate until they are all coated. Chill until chocolate hardens. Serve.: As you coat each ball, the baking sheet starts to look like a small gallery of glossy treats, the chocolate cooling into a firm, slightly shiny shell. Once they are all on the sheet, transfer them to the refrigerator for a short chill so the coating fully sets and becomes crisp to the bite. The finished aroma is a pleasant mix of roasted peanut butter and sweet chocolate. A common slip is stacking the balls before the chocolate sets, which causes sticking; always give them space and proper chill time. Serve cold or at room temperature depending on your preference, and watch how quickly they vanish.
