To make the cake: Preheat oven to 350°F (180°C). Butter one 8 x 3-Inch cake pan; line bottom of pan with parchment paper.
In a large bowl, whisk together flour, sugar, cocoa, baking soda, and salt until combined.
In a medium bowl, whisk together buttermilk, oil, eggs, and vanilla until well combined. Add buttermilk mixture to flour mixture, and whisk until smooth. Slowly add stout, whisking until fully combined and smooth. (Batter will be quite thin.) Add to prepared pan.
Bake until a wooden pick inserted in center comes out clean, about 50 minutes. Let cool completely in pan.
Remove cooled cake from pan, and place on a serving platter. Top with White Knight Frosting. Spread Whipped Cream Topping onto frosting in a smooth layer. Place a clover stencil on topping, and garnish with cocoa, if desired. Using a warm offset spatula, smooth sides of cake.
To make the frosting: In the bowl of a stand mixer fitted with the paddle attachment, beat confectioners’ sugar, Kerrygold butter, cocoa, salt, and vanilla at medium speed until smooth. Add the melted, cooled chocolate and add enough Irish whiskey, while beating, to make a smooth, spreadable frosting. Beat until combined and smooth, stopping to scrapes sides of bowl. Use immediately.
To make the cream topping: In the bowl of a stand mixer fitted with the whisk attachment, beat all ingredients at medium speed until medium-stiff peaks form.