Preheat and prepare – Set your oven to 350 °F (175 °C) and line your baking sheets.

Cream the base – In a large bowl or stand mixer, beat together softened butter, creamy peanut butter, oil, granulated sugar, and powdered sugar. Sprinkle in baking soda, cream of tartar, and salt, and beat 1–2 minutes until fluffy and light.

Add wet ingredients – Mix in sour cream, eggs, and vanilla until the mixture is smooth and uniform.

Mix in the flour – Slowly incorporate the flour, stirring just until the dough is fully combined and no dry bits remain. Avoid overmixing—stop as soon as everything comes together.

Form the cookies – Measure out 2 tablespoons of dough per cookie, roll into smooth balls, and arrange them on your prepared baking sheets with enough room to spread.

Flatten with sugar – Lightly coat the bottom of a glass with cooking spray, dip it into the extra granulated sugar, and gently press each dough ball to about ¼‑inch thickness.

Bake – Cook for 7 to 8 minutes, until the edges are just firm and the centers remain slightly soft.

Cool briefly – Allow cookies to cool on the baking sheet for a few minutes, then transfer them to a wire rack to cool completely.

Make the frosting – In a clean mixing bowl, blend softened butter, sour cream, and vanilla until smooth.

Frost – Add powdered sugar, cocoa powder, and cream, beating until fluffy and spreadable. Add more heavy cream as needed to loosen the frosting to a spreadable consistency.

Finish & serve – Once the cookies are fully cooled, generously frost each one. Serve immediately or store for later.