Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans, then line the bottoms with parchment paper. Set aside.

In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt until well combined.

Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat with an electric mixer on medium speed for 2 minutes, scraping down the sides of the bowl as needed.

Carefully pour in the boiling water (or hot coffee) and mix on low speed until just combined. The batter will be thin; this is normal.

Divide the batter evenly between the prepared cake pans. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs, not wet batter.

Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely. Do not frost warm cakes.

In a medium saucepan, combine the diced strawberries, granulated sugar, cornstarch, and lemon juice. Mix well.

Cook over medium heat, stirring constantly, until the mixture thickens and comes to a gentle boil. This should take about 5-7 minutes. Once thickened, remove from heat and let cool completely. The filling will thicken further as it cools.

In a large mixing bowl with an electric mixer, beat the softened butter on medium speed until light and fluffy (about 2-3 minutes).

Gradually add the sifted powdered sugar and sifted cocoa powder, beating on low speed until combined, then increase to medium-high and beat until smooth.

Add the heavy cream (or milk), vanilla extract, and salt. Beat on medium-high speed until the buttercream is light, fluffy, and smooth. If it's too thick, add more cream 1 teaspoon at a time; if too thin, add more powdered sugar.

Once the cake layers are completely cool, level them with a serrated knife if necessary.

Place one cake layer on your serving plate or cake stand. Spread about 1/2 cup of chocolate buttercream evenly over the top.

Pipe or spread a dam of chocolate buttercream around the outer edge of the first cake layer on top of the buttercream. This will hold the strawberry filling in place.

Fill the center of the buttercream dam with all of the cooled strawberry filling.

Carefully place the second cake layer on top, pressing down gently.

Frost the entire cake with the remaining chocolate buttercream, creating a smooth finish or decorative swirls as desired.

Garnish the top of the cake with fresh strawberries, whole or sliced, just before serving. For best results, chill the cake for at least 30 minutes before serving to allow the buttercream to set.