Make the Coffee Sponge:
Preheat oven to 350°F. Grease three 8-inch round cake pans and line bottoms with parchment. Whisk together flour, baking powder, baking soda, and salt in a medium bowl. Combine hot coffee and instant espresso powder, stirring until dissolved. Add buttermilk and set aside to cool to room temperature. In a stand mixer with paddle attachment, cream butter and sugar until very light and fluffy, 4-5 minutes. Add eggs one at a time, beating well after each addition. Add vanilla and mix until combined. Add flour mixture in three additions, alternating with coffee-buttermilk mixture, beginning and ending with flour, mixing on low just until combined after each addition. Divide batter evenly among prepared pans, about 14 oz each. Smooth tops. Bake 25-28 minutes until a toothpick inserted in center comes out clean and cakes spring back when gently pressed. Cool in pans 10 minutes, then turn out onto wire racks to cool completely.

Make the Coffee Soak:
Combine espresso, coffee liqueur, and sugar. Stir until sugar dissolves. Set aside.

Make the Coffee Mascarpone Cream:
In a stand mixer with whisk attachment, combine mascarpone, heavy cream, powdered sugar, dissolved espresso, vanilla, and salt. Beat on medium-high until mixture holds stiff peaks, 2-3 minutes. Do not overbeat or mascarpone will become grainy. Stop as soon as stiff peaks form. Refrigerate until ready to use.

Make the Vanilla Swiss Meringue Buttercream:
Ensure butter is softened but still cool; it should hold its shape but yield easily to pressure. If your kitchen is cold, let butter sit at room temperature for 1-2 hours. The ideal temperature is 65-68°F. Combine egg whites, sugar, and salt in the bowl of a stand mixer. Set over a pan of simmering water; bowl should not touch water. Whisk constantly until sugar dissolves and mixture reaches 160°F; it should feel completely smooth when rubbed between fingers. Transfer bowl to stand mixer fitted with whisk attachment. Beat on high until stiff glossy peaks form and bowl is cool to touch, 10-12 minutes. Switch to paddle attachment. With mixer on medium-low, add butter one tablespoon at a time, beating well after each addition. If buttercream looks curdled, keep beating; it usually comes together. If still curdled after 5 minutes, warm bowl briefly with a kitchen torch or over warm water, then beat again. If buttercream is soupy, butter was too warm; refrigerate 15 minutes then beat again. Once smooth and fluffy, add vanilla bean paste and vanilla seeds if using. Beat until combined.

Assemble the Cake:
Level cake layers if domed using a serrated knife. Place first layer on cake plate or board. Brush generously with coffee soak; the sponge should be moist but not soggy. Spread a generous layer of coffee mascarpone cream, about 1 cup, leaving a small border. Place second layer on top. Repeat soaking and mascarpone cream. Top with final layer, bottom-side up for a flat top. Brush with remaining coffee soak. Apply a thin crumb coat of vanilla buttercream over the entire cake. Refrigerate 30 minutes until firm. Apply remaining buttercream in an even layer, creating a smooth finish or decorative swoops.

Finish the Cake:
Dust the top generously with cocoa powder through a fine-mesh sieve; this is the signature tiramisu look. Arrange chocolate shavings or curls on top. Garnish with chocolate-covered espresso beans and cocoa nibs if using. For the cleanest slices, refrigerate cake 1 hour before cutting.