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Grease two 8 or 9 inch cake pans with butter and dust with flour.
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Sift the flour, baking powder and salt together and keep aside.
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Beat butter in handheld mixer or stand mixer until smooth and creamy (1 minute).
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Add both sugars and beat in high speed for 3-4 minutes until creamed.
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Scrap the sides and mix with spatula from bottom to top.
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Add eggs and yolks one at a time with the mixer running in low speed.
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Once done with adding eggs do add vanilla extract and beat on medium to high speed until combined.
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Butter and eggs may sit in the bottom if using the stand mixer so lift them up as needed.
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Add the dry ingredients mixture in batches by running the mixer at low speed altering with buttermilk.
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Scrap and check for big lumps any, if so continue beating to smooth consistency and don't over beat.
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The cake batter looks slightly thick unlike regular cake batter (Pic 1).
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For mild chocolate swirl like mine do take 3/4 the cup yellow batter (Pic 1 & 2) or 1 Cup for little more chocolate swirls.
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Melt the chocolate in microwave or double boiler add the chocolate melt (when its warm and not so hot) to the yellow cake batter and mix until combined.
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Now in the prepared pans divide the yellow cake batter.
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Level and tap it before adding chocolate batter.
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The chocolate batter harden/sets quickly so add and swirl when they are gooey (Pic 4, also refer notes when it hardens).
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Make patterns as you like (Pic 2) and bake in preheated oven for 22-25 minutes or until toothpick inserted comes out clean (Pic 3).
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Cool on a wire rack for an hour and wrap them tight as shown in bottom layer in Pic 5 above.
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Store them in room temperature for to use it the next day.
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Or freeze it for later use, do thaw properly before icing it.
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For that perfect buttercream shown in picture 7 and 8, recipe follows.