Sift together the flour, cocoa powder, baking powder, baking soda, and salt. In a measuring cup, combine the milk and brewed coffee. In a stand mixer, using a flat beater on medium-high speed, beat the butter and sugars together until light and fluffy, about 2-3 minutes.
To make buttercream
In a large heatproof bowl, combine the egg whites and sugar. Set the bowl over simmering water in a saucepan and whisk constantly until sugar is completely dissolved (160F), about 2 minutes. Remove from heat. In a stand mixer using a whisk attachment, whip egg white mixture on high until it holds stiff peaks, about 5-6 minutes.
Switch to flat beater and add the salt. Add butter a few at a time, beating well after each addition. Don’t worry if buttercream starts to look curdled. Just keep beating on until buttercream is smooth and creamy, about 3-5 minutes. Add melted white chocolate to the buttercream, mixing until combined.
For mocha cupcakes
For white chocolate buttercream