In a large mixing bowl, beat the butter, oil, and sugars until combined.
Beat in eggs and vanilla.
In another bowl, whisk together the flour, baking soda, and cream of tartar.
Add dry ingredients to wet in two additions, stirring just until combined. Do not overmix.
Line the pressure cooker steamer rack with parchment and drop rounded tablespoons of dough onto it.
Set cooker to bake mode or low pressure for 10 minutes to bake cookies through steam.
Do a quick release and carefully remove the cookie tray.
Cool cookies on wire racks. Let sit 5 minutes before serving.
Notes
Use parchment paper cutouts in the pressure cooker rack to prevent sticking. Chill dough ahead to improve texture. For variety, add lemon zest, nuts, or sprinkles.