Using a small sauce pot, over medium heat, simmer milk until it's hot, about 5 minutes, make sure it doesn't burn. Mix together cocoa with brown sugar in a bowl, then add hot milk gradually and stir until smooth. Set aside to cool. Make sure this mixture has cooled before adding to the butter mixture.
Preheat oven to 350º F. Grease 2 round baking pans or add parchment paper to the bottom and grease sides of pans and dust with flour.
Using a stand mixer, beat butter and sugar until light and fluffy about 5 minutes. Add vanilla, then add eggs one at a time, beat well after each addition.
Sift together flour, baking powder and baking soda in a medium bowl.
On low speed, add flour mixture to cream mixture, alternating with the cocoa mixture. Start with flour and end with flour, mix until smooth.
Pour batter into the baking pans, making sure you distribute evenly. Bake for 25-35 minutes.
Frosting: In a small sauce pot, over medium-low heat, whisk together cold milk, flour and salt until there are no lumps. Stirring occasionally until the mixture thickens, about 5 minutes. Set aside to cool.
Cream together butter, vanilla and sugar. Add the cooled milk mixture and cocoa. Beat for 10 minutes until the frosting is smooth.
Decorate cake with dark chocolate shavings, coconut, Easter eggs etc.