These charming and nostalgic sweets can be served family-style.
Preheat oven to 350 degrees. Generously butter 9 mini loaf pans. Sift together flour, baking powder, and salt.
Beat butter and 1 cup sugar with a mixer on medium-high speed until pale and fluffy, about 5 minutes. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Reduce speed to low. Beat in flour mixture in two additions, alternating with the buttermilk. Reserve one-quarter of the batter.
Whisk together cocoa and remaining 1 tablespoon sugar. Add the boiling water, and stir until smooth. Add to reserved cake batter, and stir until combined.
Spoon batters into pans in two layers, alternating spoonfuls of vanilla and chocolate to simulate a checkerboard. Run a skewer through batter in a swirling motion to create a marbled effect.
Bake cakes, rotating pans from front to back after 15 minutes, until a toothpick inserted into centers comes out clean, 25 to 30 minutes. Let cakes cool in pans on wire racks for 15 minutes. Run a knife around edges of pans to loosen, and turn out cakes onto racks to cool completely. (Cakes can be wrapped in plastic and stored at room temperature for up to 3 days.) Just before serving, cut cakes into 1/2-inch slices.