Beat butter and sugars with an electric mixer on medium speed until light and fluffy, about 4 minutes.
Add eggs and egg yolks 1 at a time, beating on medium-low after each addition.
Add vanilla; beat just until incorporated.
Whisk together flour, baking powder, and salt in a medium bowl.
Add to butter mixture alternately with sour cream, beginning and ending with flour mixture, beating on low speed just until combined after each addition.
Divide Basic Batter evenly between 2 large bowls.
Pumpkin Spice Batter
Add flour, canned pumpkin, and pumpkin pie spice to 1 of the Basic Batter bowls; stir together until completely combined. Set aside.
Chocolate Batter
Add cocoa, sour cream, and instant coffee granules to remaining Basic Batter bowl, and stir until smooth.
Drop large spoonfuls of the Pumpkin Spice Batter across the bottom of eight 5 ¾" x 3 ¼" mini loaf pans.
Repeat with Chocolate Batter.
Swirl batters with a long pick, moving back and forth from one end of the pan to the other.
Bake in a preheated oven for 25 minutes or until a wooden pick inserted into the center of a loaf comes out clean. Cool on a wire rack.